Task 2 - Real Engines a. Study relevant articles on engine performance and write

Question

Task 2 - Real Engines a. Study relevant articles on engine performance and write a report explaining what limits the compression ratio in real engines. You should find out about the following: i. What are typical compression ratios? i. What are the maximum temperatures and pressures that a modern engine can withstand? ili Why are real engines less efficient than predicted by the Otto formula? iv. What are the main energy losses? b. Write a report on the following methods of improving engine performance. Explain the differences they make to engine performance and why they produce improvements i. Supercharging ii. Turbocharging ii. Intercooling

Explanation / Answer

a)

1)

Compression ratio is defined as the volume at below dead center divided by volume at the top dead center.

The compression ratio of petrol engine varies from 6:1 to 10:1 The higher the octane number, higher the compression ratio we can set. The compression ratio of diesel engine ranges from 16:1 to 20:1, higher than petrol engines.

2)

Modern engines can withstand the temperature range of 1400 C. the pressure range is 300 psi for engine's at light loads, to 1000 psi for production engines at full power to 1500 psi or greater for race engines.

3)

1)The actual cycle does not have the sharp transitions between the different processes that the ideal cycle has, so actual engine yield less efficiency than otto engine.

2) In Otto cycle, we consider that there are no heat losses. In the actual engine, there is heat loss to surroundings, we supply cooling water to keep cylinders preventing from melting off, so it absorbs energy which makes less efficient than Otto cycle efficiency.

3)In the theoretical cycle, it is assumed that combustion takes place at constant volume which not true in the actual cycle.

4) In the actual cycle, the specific heats increase with an increase in temperature, whereas in Otto cycle, we consider them as constant.

5) we do not consider any frictional losses in Otto cycle, whereas frictional losses present in real cycle make them less efficient than Otto efficiency.

4)

Main energy losses are frictional losses and heat lost to cooling losses in IC engines.

b)

1)The amount of power any engine produces depends on how much air/fuel mixture is pumped through it in a given instant of time.

as RPM go up, volumetric efficiency drops off. The only way to overcome this limitation is to add a forced induction system. By mechanically increasing the volume of air that enters the engine, forced induction in effect increases atmospheric pressure giving the air more "push" as it enters the cylinders. Volumetric efficiency goes up and the engine makes substantially more power. Now the engine can breathe at 100 percent volumetric efficiency or higher depending on how much "boost" is built into the system.

2) turbocharging:

A turbocharger is used to increase the fuel efficiency without increasing power. Waste energy from engine exhaust is made to recirculate and is used to heat intake charge. So Increase in the temperature of the charge enhances fuel efficiency. Remember that excess intake air temperature lead to engine knocking and reduces engine efficiency. So one has to make a good compromise in it.

3) Intercooling:

The turbocharged fresh charge will have a higher temperature, so this excess air temperature reduces engine efficiency. So Intercooler is adapted in order to reduce air temperature to the suitable range. With a decrease in air temperature air density decreases and volumetric efficiency in increased.

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