Question

In company that designs road cars and heavy trucks, it important to make the proper selection of the engines. Your part as a system energy engineer is to make a proper selection between an engine operates on Otto cycle/gasoline engine or Diesel cycle/diesel engine. Your head of department asked you to provide a detailed technical report that shows your selection for engine to be used with heavy truck vehicle. This is best achieved by performing the following tasks: Section.1:

1. Which engine does operate at lower ??? and has higher air-fuel mass ratio?
2. Which engine does burn fuel more completely?
3. Which engine has lower fuel cost?
4. Explain what compression ratio for Otto and Diesel cycles means
5. .Explain what cut-off ratio for Diesel cycles means

Answer 1

Which engine does operate at lower ??? and has higher air-fuel mass ratio?

Diesel engine operates at lower rpm, because compression stroke takes time for ignition.

Petrol engine has high A/F mass ratio of 14.7:1 than diesel engine 14.5:1.

Which engine does burn fuel more completely?

Diesel engine burns fuel more completely because of higher compression ratio.

Which engine has lower fuel cost?

Diesel enginer operates on Diesel oil, which is cheaper than Petrol.

Explain what compression ratio for Otto and Diesel cycles means

Compression ratio is defined as ratio of maximum volume (BDC) to minimum volume (TDC) of engine.

Compression ratio of petrol engine is limited to 8-10, while for diesel engine it is 14 - 25.

It is limited in petrol because if we increase the compression atio the fuel will auto ignite before the sparking occurs and causes knocking in the engine.

It is more in diesel engine, because air is compressed so that it crosses self igntion temperature of diesel so it can auto ignite without need of external sparking.

Explain what cut-off ratio for Diesel cycles means

It is ratio of volume of cylinder at end of heat addition to the beginning of heat addition.

Higher cutoff ratio means high amount of heat addition, which decreases the thermal efficiency of engine.

From the above points we can conclude that heavy trucks will require diesel engine because of high torque (low rpm) , high efficiency and lower cost of fuel.