Posted tagged ‘drive shafts’


September 13, 2011


The first decision in designing an engine installation is selection of the coupling and drive method to connect the engine to the driven equipment..
The coupling and drive selection connections are closely related to the proper selection of engine support and mounting. This ensures a successful trouble-free installation from the standpoint of both the engine and driven equipment, as well as the power transmission components. (Refer to Mounting and Alignment section.)

A rigid precision-type mounting system must be provided for both the engine and driven equipment if a solid or nearly solid driveline is utilized.

Drive components which utilize universal joints, drive shafts or belts, and chain-type drives permit slightly greater alignment deviations.

When selecting the power transmission system, the possible need for a complete torsional analysis must be considered. System incompatibility will result in premature and/or avoidable failures.Refer to Mounting and Alignment section

General Description and Selection Considerations
Engine starting capability is normally limited and the direct connection of large mass
driven equipment makes starting difficult or impossible, therefore, a type of clutch or
disconnect device may not only be desirable but necessary.

Exceptions, if properly sized to the engine starting capability, may be centrifugal pumps, fans or propellers, and generators which provide a direct connected load with
a low starting torque requirement. Certain compressors which utilize a starting “unloading device” may also be direct connected.

Piston-type pumps, most compressors, belt- and chain-driven equipment, and all mobile vehicles will require an engine disconnect system.

The engine disconnect feature provides an important safety and service function. It permits rotating the engine for service and adjustment, as well as servicing the driven
equipment without disconnecting the drive-train. It also permits engine warm up before applying load — an accepted requirement for extended engine life. On multiple engine installations driving into a common compound or driven machine, it permits operating at less than full power level if desired, as well as at partial power should one engine be down for routine service or because of failure.

Numerous devices are available for connection or engagement of the engine to the driven machine. The device selection will depend on the desired engagement function; however, several general considerations must be made regardless of the
device selected.

The selected device must have adequate capacity to transmit the maximum engine
torque to the driven equipment. With the exception of “dog-type” clutches, which are
generally not acceptable on material handling equipment, clutches rely on friction
for power transmission.
(Dog-type clutches provide a direct mechanical connection and cannot be engaged
during operation nor do they have any modulating [slipping] capability).

Engine-Mounted Enclosed Clutches
These clutches (power takeoffs) will be covered in greater detail under the following
classifications (clutch rating definitions), as well as the specific selection considerations for the type of clutch and application.

Enclosed clutch selection for either rear or front engine mounting must be made in
accordance with the “Horsepower Absorption Capability”.

The following rating definitions are applicable to clutch arrangements offered by

Light-Duty (LD)
A light-duty clutch is used primarily to disconnect and pick up light inertia loads, but
does more work during engagement than “cut-off” duty.

A light-duty clutch should engage within two seconds, start the load less than six times per hour, and never heat the pressure plate outer surface above hand holding temperature.

Example: Disconnect clutch between engine and hydraulic torque converter with engine above low idle when engaging clutch, as in power shovel master clutch, generator, or similar drives.

Normal-Duty (ND)
A normal-duty clutch is used to start inertia loads with frequencies up to 30 engagements per hour. More important is that the clutch can start the heaviest inertia load within three seconds, and that the product of seconds of clutch slip per engagement times number of engagements per hour be under 90.

A normal-duty application may raise the outer clutch surface temperature to under
100°F (37.8°C) rise above ambient air temperature.

Example: Power takeoff starting average inertia loads where starting load is 40% of the running load.

Heavy-Duty (HD)
A heavy-duty clutch is used to start inertia loads with frequencies up to 60 engagements per hour. More important is that the clutch can start the heaviest inertia loads within four seconds, and that the product of seconds of clutch slip per engagement times number of engagements per hour be under 180.

Heavy-duty applications may raise the clutch outer surface temperature to a maximum of 150°F (65.6°C) rise above ambient air temperature.

Example: Power takeoff starting average inertia loads whose starting load is 80% of
the running load. Also, rock crusher applications where the clutch is not used to
“break loose” jammed loads.

Extra Heavy-Duty (EHD)
An extra heavy-duty clutch is used to start inertia loads requiring over four seconds to start the heaviest load, with longest slip period per engagement not exceeding 10seconds. Also, when the product of seconds of clutch slip per engagement times number of engagements per hour exceeds 180, it is beyond extra heavy-duty. Contact your Caterpillar dealer for application approval of extra heavy-duty-type service.
Example: Power takeoff starting inertia loads whose starting load approaches or
exceeds the running load.

Typical Light-Duty (LD)
Clutch Applications
A.Agitators — pure liquids.
B.Cookers —cereal.
C.Elevators, bucket — uniform loads,
all types.
D.Feeders — disc-type.
E.Kettle — brew.
F.Line shafts — light-duty.
G. Machines, general — all types with uniform loads, nonreversing.
H.Pumps — centrifugal.

Typical Normal-Duty (ND)
Clutch Applications

A.Agitators — solid or semisolids.
B.Batchers — textile.
C.Blowers and fans — centrifugal andlobe.
D.Bottling machines.
E.Compressors — all centrifugal
F.Elevators, bucket — uniformly loaded or fed.
G.Feeders — apron, belt, screw, or vane.
H.Filling machine — can type.
I.Mixers — continuous.
J.Pumps — three or more cylinders; gear- or rotary-type.
K.Conveyor — uniform load.

Typical Heavy-Duty (HD)
Clutch Applications

A.Cranes and hoist — working clutch.
B.Crushers — ore and stone.
C.Drums — braking.
D.Compressors — lobe rotary plus three or more cylinder reciprocating-type.
E.Haulers — car puller and barge-type.
F.Mills — ball-type.
G. Paper mill machinery — except calenders and driers.
H.Presses — brick and clay.
I.Pumps — one- and two-cylinder reciprocating-type.
J.Mud pumps — one- and two-cylinder reciprocating-type.

Typical Extra Heavy-Duty (EHD)
Clutch Applications

A.Compressors — one- and two-cylinder reciprocating-type.
B.Calenders and driers — paper mill.
C.Mills — hammer-type.
D.Shaker — reciprocating-type.

Once all machine parameters have been established, contact your Caterpillar dealer
for selection assistance.

Automotive-Type Clutches
Also known as diaphram or spring-loaded-type clutches, this category is generally a
light-duty classification; it is normally used in strictly mobile applications, such as on-
highway trucks or higher speed mobile machines, which utilize a multi speed transmission. The automotive-type clutch is normally foot-operated for disengagement or is engaged with the friction being generated by spring force acting on an engine-driven plate.

Although this type of clutch is not a Caterpillar price list attachment, on the
smaller engine families, there is offered a selection of flywheels to accommodate the
more common commercial models offered by a number of manufacturers.

If the machine design requires this type of clutch, the package designer and installer
should work very closely with the clutch manufacturer to ensure proper selection.


Air Clutches
Basically, engagement friction is maintained by air pressure. This feature is particularly advantageous when remote control of the engagement/disengagement functions is required.

Air clutches utilize an expanding air bladder for the clutch element. (See Figure 3).

Air clutches do not normally have side load capability, so if such capability is required, the output shaft must be supported by two support bearings. These bearings must be mounted on a common base with the engine package. Air pressure to operate the clutch is supplied by an air connection through the drilled passage in the output shaft. Clutch alignment tolerances are reduced as air pressure to the clutch increases.

When selecting an air clutch, the package designer/installer must work closely with the clutch manufacturer.

Centrifugal Clutches
The centrifugal clutch accomplishes the engagement/disengagement functions by centrifugal force which is generated by the engine operating speed. It provides a power engagement/disengagement function controlled strictly by the engine governor speed control (throttle).

Centrifugal clutches offer smooth automatic engagement of load without complicated
controls. Typically, a diesel engine with a full load operating speed of 1800 rpm will
be fitted with a centrifugal clutch which effects engagement at a speed of about
1000 engine rpm. Once engaged, most clutches of this type will remain engaged
even if the engine speed is pulled down due to load — as low as the engagement
speed (i.e., 1000 rpm) or lower (e.g., disengagement at 800 rpm). If the load is
such that engine stall speed is approached, the clutch will disengage.

As with the air-type clutches, they have limited or no side load capability and
for other than in-line drive loads, a separately supported output shaft with two support bearings must be provided and must be mounted on a common base with the engine package.

When selecting a centrifugal clutch, the package designer/installer must work closely with the clutch manufacturer.

Over the years rapid technological advances have enabled numerous commercial
manufacturers to offer a broad range of transmissions with nearly unlimited features and options.

For this discussion transmissions will be divided into three broad classifications all
of which transmit power through sets of mechanical gears, either spur or helical
types, or planetary designs. Where multi-speed capability is provided, it is accomplished either mechanically or automatically (hydraulically, pneumatically, etc.)

Due to the large number of transmissions commercially available and the fact that
Caterpillar does not offer transmissions (with the exception of marine transmissions —single speed — forward/reverse functions(,the transmission discussion will be restricted to general operating principles and considerations.

When selecting a transmission, the package designer must work closely with the
transmission manufacturer.


Mechanical Transmission
The mechanical transmission provides the lowest cost method of providing multiple
output speeds when the driven equipment input speed range or torque requirements
exceed the operating capability of the diesel engine. Mechanical transmissions are usually equipped with some type of clutch assembly to facilitate not only engine starting but also to change gear ratios.

This type of transmission is applicable to both semi mobile and mobile installations
where the momentary loss of power to the driven equipment when gear changes are
effected does not pose operating problems.

Generally, the mechanical transmission is employed when the gear speed change
requirements are not a constant require- ment and the speed shifts do not have to
be executed rapidly.

Today’s modern mechanical transmission, when properly matched to the engine-driven equipment, will provide reliable trouble-free service. Frequent gear changes,
however, will accelerate clutch wear and maintenance costs.

Installation is simplified since mechanical transmissions do not normally require oil
cooling systems as do the automatic type.

Automatic, Semiautomatic, and Preselector-Type Transmissions
As the names imply, these transmission types effect the gear changes either completely automatically or as predetermined by the machine operator.
Engine power engagement/disengagement clutching is normally fully automatic and
does not require the machine operator to physically move a clutch pedal or lever. For
disengagement the operator need only move the selector lever to a neutral position.

As with the mechanical transmission, the automatic type must be carefully matched
to the engine operating horsepower, torque, and speed characteristics. However, with the automatic types, additional match consideration may be required since they normally utilize a torque converter, hydraulic coupling, or other type of non mechanical engagement device for the power engagement/disengagement function.

This is nearly always accomplished hydraulically. The automatic-type transmissions provide operator ease of machine operation, as well as a nearly constant power flow to the driven equipment during gear changes.

A number of commercial manufacturers offer a wide range of automatic-type transmission. The package designer/installer must work closely with the transmission
supplier to ensure the transmission properly matches the machine application and provides the desired operating features.

Some automatic transmission designs utilize a lockup feature. This device, in effect,
turns the transmission into a direct mechanical drive to eliminate the inherent inefficiencies of the hydraulic clutching device.

Generally, the higher cost of an automatic transmission can be justified with a machine requiring high productivity and frequent load cycle changes.

When using automatic-type transmissions, other installation considerations are required since most types require a system to cool the transmission oil. Caterpillar offers jacket water connections to supply cooling water to customer or transmission manufacturer-supplied heat exchangers.

Also offered are complete heat exchanger packages, but care must be exercised to
ensure that the Caterpillar system is capable of handling the transmission heat rejection. The cooling system capacity of the systems offered by Caterpillar can be obtained from your Caterpillar dealer and is in the Owner’s Maintenance Manual.

Speed Increasers/Reducers
These power transmission devices resemble a mechanical transmission in that power is normally transmitted through a mechanical gear set of spur or helical gears. They are used when the engine speed range is not compatible with the driven equipment input speed requirements and when the installation is best suited to an in-line drive arrangement rather than the offset belt of chain drive systems.

Speed increasers/reducers generally utilize a mechanical cutoff clutch for engine starting and are usually of a single-speed, non reversing design, although exceptions
to the above do exist. They seldom exceed two speed ratios.

Speed increasers/reducers are available for either direct engine mounting or for remote mounting. The remote-mounted type should be on a rigid common base with the engine for ease of alignment.
The package designer/installer must work closely with the commercial gear supplier to ensure proper selection and installation.

Although infrequently found in material handling/agriculture applications, specific de-signs may require an engine compound.

Basically, a compound is an enclosed gear or chain device which permits several engines to provide input power with the power output coming from one or more shafts.

Compounds providing a single engine input and multiple outputs is most common. An example would be a hydrostatic machine where a single engine provides power to
multiple hydraulic pumps when separate pumps are used for the various functional drives of the machine.

Multiple engine compounds can be used in applications where less than the installed horsepower capability is occasionally called upon for part load operation of the driven machine.

When part load operation is adequate, the excess capability can be removed by
declutching engines, reducing overall operating costs and maintenance.

The package designer/installer must work
closely with the compound manufacturer
to ensure proper selection and installation.

Stub Shafts
Where the application permits, a stub shaft will provide a low cost, simple method of
direct power transmission.

Stub shaft drives must not be used when the starting load of the driven equipment is
sufficient to impair engine starting unless a declutching or unloading device is utilized.
Stub shafts also have limited side load capability.

Hydraulic drive
Hydraulic drive devices generally fall into two major classifications: fluid or hydraulic couplings and torque converters.

The theory involved is similar in all types of hydraulic drives although the internal
design may vary. Basically, the engine output is absorbed by a turbine-type pump.

The oil or fluid in the pump housing is accelerated outward, and the engine power is
transmitted to the outer edge of the pump as kinetic energy in the form of high velocity fluid. This energy is then transferred back towards the center of the output
shaft. This is where the differences occur between a hydraulic or fluid coupling and a
torque converter.

Fluid (Hydraulic) Couplings
In the fluid couplings, the high velocity fluid is directed into a matching turbine located very close to the turbine-type pump which is engine driven. The matching turbine absorbs the energy as the fluid is directed back toward the center of the coupling and the energy is delivered to the output shaft.

The output torque will always equal the input torque less internal friction losses which will be observed as a lower output speed (rpm)than the input speed (engine rpm).

The primary advantage of a hydraulic coupling is the total lack of a mechanical connection between the driving engine and the driven equipment.
This isolates or greatly reduces the transfer of mechanical shocks, vibration, and undesirable torsional effects between the driven load and the engine.

A hydraulic coupling will prevent engine stall under load; however, the engine can be pulled down in speed by varying degrees depending on the hydraulic coupling fluid
cooling capacity. It also permits starting high inertia-driven loads without the use of a cut-off clutch.

The main disadvantages of a hydraulic coupling are the reduced efficiency over a mechanically coupled drive and its inability to generate a torque multiplication as is
possible with a torque converter.

Normally, hydraulic couplings are best suited to applications which are constant speed applications where the slip capability is desirable to compensate for shock loads, overloads, high inertia load startups, and assist in torsional vibration reduction.
Torque Converters
As with hydraulic couplings, torque converters differ considerably in internal construction and refinement but can generally be placed in two classifications: single-stage and multistage. These differences will be expanded later in this section.

The torque converter differs from the hydraulic coupling in that one or more third
members, called stators or turbine reactors, are utilized in addition to the input pump and the output turbine. These stators or reactor members are imposed in the fluid flow path in such a manner as to produce a multiplication of the input torque to the output shaft at reduced output speeds (rpm).

The maximum torque is transmitted to the output shaft (driven equipment) at stall condition (output shaft is not rotating) when it will equal from 1.6 to more than 6.0 times the converter input torque (engine output torquevalue). When operating at full
speed, with the imposed load at a level which permits the output speed to be close to the engine speed, the torque converter acts in principle like a hydraulic coupling.

The necessity of matching a torque converter to the engine cannot be overemphasized. An improperly sized converter, one with the wrong blading or one which operates in a highly inefficient speed range, will prove unsatisfactory. An improperly matched torque converter can result in engine over- load, high inefficiency, high fuel consumption, poor engine response, and other undesirable results.

The torque converter manufacturer generally has computer programs which, when
coupled to the performance characteristics of the engine, can ensure a correct “match” for any installation/application. Most converter manufacturers have performance data on the Caterpillar Diesel Engine models or data can be obtained from your Caterpillar dealer. This data is covered in the Caterpillar Technical Information File (TIF). Performance data for nonstandard ratings is also available from your Caterpillar dealer.

Additionally, cooling of the torque converter fluid is required. Torque converter cooling must be provided for the equivalent of at least 30% of the total engine heat rejection when using a pre combustion chamber-type engine. When using a direct injection-type engine, torque converter cooling must be provided for the equivalent of at least 50% of the total engine heat rejection.
Caterpillar offers, as price list attachments, either jacket water connections for heat
exchanger-type coolers or, on the 3200,3300 and 3400 Series Engines, complete heat exchanger cooling packages. It is imperative that the cooling package be of adequate capacity. The capacity of

Most commercially available converters are also offered with attachment cooling

If the engine cooling system is used to cool the torque converter, adequate reserve
radiator capacity must be provided.

Single-Stage Torque Converters
This type of converter is normally selected for light-duty applications. It has a decreasing torque absorption curve as the output speed approaches stall condition and will not pull down the engine input speed (lug the engine).

Multistage Torque Converters
Most applications will utilize a multistage converter. They provide a broader usable
range and higher torque multiplication value than single-stage converters.

Torque converter manufacturers provide excellent manuals and assistance in the
selection of the correct converter for a specific application. Consequently, rather than elaborating on selection guidelines in this publication, it is suggested that the package designer/installer counsel with the converter manufacturer for expert advice.
In addition to offering the same benefits as a hydraulic drive, the torque converter also offers a torque multiplication benefit as well as, if properly matched, higher power transmission efficiency. The multistage converter is particularly preferred for variable output speed applications.

As standard price list attachments, Caterpillar offers flywheels to couple to most commercial torque converters and hydraulic drives.

Special Considerations
With the selection of any of the above methods of power transmission, several
general areas must also be given special consideration to ensure a successful

Side Loading
Excessive side loading is one of the most commonly encountered problems in the
transmission of engine power.

It is impossible to overemphasize the need for accurate evaluation of side load imposition on all types of power transmission devices.

For Caterpillar-supplied attachment power takeoffs, the Caterpillar Industrial Engine
Price List LEKI8162 provides complete instructions and capacity data for side load
For power transmission devices supplied by others, the manufacturer must be consulted for a capability analysis of his equipment.

Overhung Power Transmission Equipment
Power transmission equipment, which is directly mounted to the engine flywheel housing, must be evaluated to ensure that the overhung weight is within the tolerable limits of the engine. If not, adequate additional support must be provided to avoid


The dynamic load limits and the maximum bending moment that can be tolerated by
the flywheel housing can be obtained from your Caterpillar dealer.

For determination of the bending moment of overhung power transmission equipment
installations, see Figure 13.

To compensate for power transmission systems which create a high bending
moment due to overhung load, a third mount is required. Proper design of the
support is essential. Forces and deflections of all components of the mounting
system must be resolved. If the third mount is in the form of a spring, with a vertical rate considerably lower than vertical rate of the rear engine support, the effect
of the mount is in a proper direction to reduce bending forces on the flywheel
housing due to downward gravity forces, but the overall effect may be minor at high
gravity force levels. The use of supports with a vertical rate higher than the engine
rear mount is not recommended since frame bending deflections can subject the
engine power transmission equipment structure to high forces. Another precaution is to design the support so that it provides as little resistance as possible to engine roll.

This also helps to isolate the engine/transmission structure from mounting frame or base deflection.

Wet Flywheel Housings
Certain types of power transmission equipment require a “wet” flywheel housing.
Wet housing equipment requires that the flywheel housing be able to accommodate a degree of flooding by the fluid medium of the power transmission equipment. The standard Caterpillar Diesel Engine does not:

A.Contain sufficient provisions for seal- ing in the area of the rear crankshaft
seal to prevent the transfer of the power transmission fluid into the engine lubricating oil reservoir (pan).

B.Have the capability of evacuating the transmission fluid from the flywheel
housing back to the transmission reservoir to prevent engine crankshaft seal flooding.
Unless a belt, chain, or universal joint-type drive is taken directly from the output shaft of the engine-driven power transmission device, the use of some type of mechanical coupling device is recommended.

The coupling must be installed between the power transmission output shaft and
the input drive shaft of the driven machine.

On close-coupled driven equipment, the use of a coupling can be avoided if two basic
criteria are met:
A.Is the torsional compatibility of the driven machine compatible with the engine to the point that lack of a coupling will not cause either engine or driven machine problems?

B.Is the package base sufficiently rigid to avoid any distortion during operation?
Does it contain sufficient alignment control features to successfully retain alignment during operation to preclude the need for the misalignment tolerance capability of a coupling?

Seldom can both of these questions be answered affirmatively.
A large number of commercial coupling designs, are available to the package


Commercial couplings make use of resilient materials ranging from rubber and tough
fabrics to springs and air-filled tubes and drums in order to absorb minor mechanical misalignment and relative movement between engine and load. It is important to
have the best possible alignment and put a minimum load and reliance on the flexible
coupling. Air clutches are not flexible couplings and imposing misalignment on them
will cause damage.

Four distinct characteristics must be taken into account in the selection of a suitable
A. Misalignment Capability
The coupling must be capable of compensating for any misalignment between the engine and equipment to prevent damage to the machine and/or diesel engine crankshaft and bearings.

If single bearing equipment is used, the coupling must be torsionally and radially rigid to transmit the load and support the weight of the driven equipment input shaft.
It must be flexible to compensate for angular misalignment
due to:
1-Thermal growth differences between the diesel engine and driven equipment.
2-Dimensional tolerances between the two units and dynamic conditions, such as torque reaction.
3-Momentary misalignment due to shock or other transient conditions.

B. Stiffness
The coupling must be of proper torsional stiffness to prevent critical orders of torsional vibration from occurring within the operating speed range. For single-bearing driven equipment, a complete torsional analysis is necessary to ensure compatibility. For two-bearing driven equipment, a simpler type of analysis is adequate. A complete torsional vibration analysis can be obtained from your Caterpillar Engine supplier, as can mass-elastic data on the diesel engine to permit
customer analysis.

C. Serviceability
When selecting a coupling, ease of installation and service is an important consideration. If spacers can be used to permit removal and installation of the coupling without disturbing the diesel engine driven machine alignment, time can be saved if service or replacement of the coupling is ever required.

When selecting a coupling, ensure that the design can withstand reasonable misalignment without materially decreasing the service life of the flexible elements.

When coupling design demands extremely close alignment, one of the major purposes for using a coupling is defeated.

D. Coupling Selection
In any installation, the coupling should be the weakest part of the entire power train; the first part to fail.

If failure does occur, the chance of damage to the diesel engine and driven machine is minimized. Safety measures must be considered to prevent major equipment damage should coupling failure occur. The use of a standard, commercially available coupling offers the benefit of parts avail- ability and reduced downtime in case of failure.

Many applications have a requirement for auxiliary drive capability to power charging
alternators, air compressors, hydraulic steering pumps, etc.
Caterpillar offers, as price list attachments, various auxiliary drive options for all engine models. These attachments provide either mechanical gear or belt drive capability.

Gear Drives
These drives are suitable for direct mounting of air compressors and hydraulic
pumps for power assist steering, etc.

Belt Drives
Several options exist for belt driving various auxiliary attachments. Both of the following methods are available from Caterpillar:

A. Crankshaft Pulleys
Additional stack-on pulleys can be added to the front of the crankshaft.
The number of additional grooves which can be added depends on other belt-driven equipment such as cooling fans and charging alternators and the amount of total side load which will be imposed on the front of the crankshaft.

B. Gear Drive Pulleys
The gear drive auxiliary positions may be equipped with output pulleys.


August 23, 2011

01-hydraulic hybrid system-Hydraulic hybrid vehicles-HHV-hydraulic motors to power wheels-accumulators to store the pressurized  fluid nitrogen gas 

Introduction To Hydraulic Hybrid Vehicles:

Hybrid vehicles use two sources of power to drive the wheels. In a hydraulic hybrid vehicle (HHV) a regular internal combustion engine and a hydraulic motor are used to power the wheels.

Hydraulic hybrid systems consist of two key components:

  • High pressure hydraulic fluid vessels called accumulators, and
  • Hydraulic drive pump/motors.

Working of Hydraulic Hybrid Systems:

01-hydraulic-hybrid-retrofit-hydraulic hybrid system-HHS-regenerating braking energy

The accumulators are used to store pressurized fluid. Acting as a motor, the hydraulic drive uses the pressurized fluid (Above 3000 psi) to rotate the wheels. Acting as a pump, the hydraulic drive is used to re-pressurize hydraulic fluid by using the vehicle’s momentum, thereby converting kinetic energy into potential energy. This process of converting kinetic energy from momentum and storing it is called regenerative braking.

The hydraulic system offers great advantages for vehicles operating in stop and go conditions because the system can capture large amounts of energy when the brakes are applied.

The hydraulic components work in conjunction with the primary. Making up the main hydraulic components are two hydraulic accumulator vessels which store hydraulic fluid compressing inert nitrogen gas and one or more hydraulic pump/motor units.

The hydraulic hybrid system is made up of four components.

  • The working fluid
  • The reservoir
  • The pump or motor
  • The accumulator

The pump or motor installed in the system extracts kinetic energy during braking. This in turn pumps the working fluid from the reservoir to the accumulator, which eventually gets pressurized. The pressurized working fluid then provides energy to the pump or motor to power the vehicle when it accelerates. There are two types of hydraulic hybrid systems – the parallel hydraulic hybrid system and the series hydraulic hybrid system. In the parallel hydraulic hybrid, the pump is connected to the drive-shafts through a transmission box, while in series hydraulic hybrid, the pump is directly connected to the drive-shaft.

There are two types of HHVs:

  • Parallel and
  • Series.

Parallel Hydraulic Hybrid Vehicles:

01-hydraulic hybrid cars-HLA system-pump mode to motor mode-parallel hydraulic hybrid vehicles-nitrogen accumulator pressure 5000 psi

In parallel HHVs both the engine and the hydraulic drive system are mechanically coupled to the wheels. The hydraulic pump-motor is then integrated into the driveshaft or differential.

Series Hydraulic Hybrid vehicles:

01-hydraulic hybrid vehicles-combines regular internal combustion engine- hydraulic motor as a accumulator-kinetic energy into potential energy to drive the vehicle

Series HHVs rely entirely on hydraulic pressure to drive the wheels, which means the engine does not directly provide mechanical power to the wheels. In a series HHV configuration, an engine is attached to a hydraulic engine pump to provide additional fluid pressure to the drive pump/motor when needed.


  • Higher fuel efficiency.  (25-45 percent improvement in fuel economy)
  • Lower emissions.  (20 to 30 percent)
  • Reduced operating costs.
  • Better acceleration performance.