## Posted tagged ‘vaporization’

### Mechanical Engineering Interview Questions

September 23, 2011
• What is Heat rate of a Power plant?

Heat rate is a measure of the turbine efficiency. It is determined from the total energy input supplied to the Turbine divided by the electrical energy output.

• The highest value of Thermal conductivity is expected for

a.) Solid ice   b.) Melting ice c.) Water d.) Boiling water.

Give the Thermal conductivity value.

Solid Ice – 3.48 W/mK

Melting Ice thermal conductivity value is low because temperature is less.

Water’s Thermal conductivity value is 2.18 W/mK.

• What is a Hydrostatic system?

Hydrostatics is the study of ﬂuid bodies that are

• At rest
• Moving suﬃciently slowly so there is no relative motion between adjacent parts of the body

For hydrostatic situations

• There are no shear stresses
• There are only pressure forces that act perpendicular to any surface.

It’s a closed loop hydraulic systems. It comprises of motor and pump. Here pump supplies energy to motor and motor gives return energy to pump supply.

• If you heat a piece of steel with a hole in the center will the diameter of the steel get bigger or smaller?

It gets bigger.

(Example: Always the bearings are heated first for new installation in a shaft.)

• What is the difference between Blower and Fan?

Fan is an air pushing device. Either Axial or Centrifugal type systems are used to move the air in low pressure. It is rotated by a motor separately.

When the fan is a housing of blades and motor, then it called as Blower. It directs the air in a single path with high pressure.

• How to find a Ductile-Brittle Transition Temperature in metals?

The point at which the fracture energy passes below a pre-determined point for a standard Impact tests. DBTT is important since, once a material is cooled below the DBTT, it has a much greater tendency to shatter on impact instead of bending or deforming.

• What is Hydrodynamic Cavitation?

Hydrodynamic cavitation describes the process of vaporization in a constrained channel at a specific velocity.

Bubble generation and Bubble implosion which occurs in a flowing liquid as a result of a decrease and subsequent increase in pressure.

### DISI ENGINE

August 23, 2011

In developing the DISI engine, we aimed to cool the interior of the cylinder as much as possible by promoting fuel vaporization and uniform mixing of atomized fuel and air. This produces a high charging efficiency of the air-fuel mixture and a high compression ratio, which results in significant improvements in both torque and fuel efficiency.

Characteristics of the direct injection engine:

• Fuel is injected from a tiny nozzle into a relatively large cylinder, so it has a high latent heat of vaporization, which efficiently cools the air within (in-cylinder cooling effect).

• The air temperature in the cylinder decreases, which means:

• (1) more air may be charged into the combustion chamber, which produces increased torque.

• (2) the engine is less prone to knocking. This contributes to increased torque, and enables a higher compression ratio that also contributes to good fuel efficiency.

In a direct injection engine, however, the fuel skips the waiting period it would have to endure inside a standard engine and instead proceeds straight to the combustion chamber. This allows the fuel to burn more evenly and thoroughly. For the driver, that can translate to better mileage and greater power to the wheels.

In the past, direct injection posed too many technical hurdles to make it worthwhile for mass market gasoline automobiles. But with advances in technology and greater pressure to make cars run more cleanly and efficiently, it looks as if gasoline direct injection — or GDI as it’s referred to in industry lingo — is here to stay. In fact, most of the major car manufacturers make or plan to soon introduce gasoline cars that take advantage of this fuel saving and performance enhancing system.