Petrol vs Diesel Engines. Image credit

Petrol vs Diesel Engines. Image credit

ZIMSEC O Level Combined Science Notes: Efficiency: Diesel vs Petrol engines

  • Efficiency refers to the fuel economy of an engine
  • It measures the amount of chemical energy in the fuel that is converted to kinetic energy
  • The efficiency of Petrol engines is around 25%
  • Diesel engines have an efficiency of around 40%
  • This means diesel engines have a higher fuel efficiency when compared to Petrol engines
  • That is a car with a diesel engine will in theory travel longer per each litre of fuel when compared to a Petrol car
  • Efficiency can thus also be measured in fuel economy terms
  • Both diesel and petrol engines however cause pollution due to incomplete combustion of their respective fuels
  • This is because the air supply in the engines is limited
  • Carbon (soot) and carbon monoxide are produced
  • Diesel engines produce less carbon monoxide as they are more efficient

How to improve the efficiency of diesel and petrol engines?

  • Cleaning/Replacing blocked and old air filters regularly to ensure an adequate supply of air
  • Ensuring the choke is functioning properly
  • Cleaning/Replacing blocked and worn out fuel jets
  • Ensure that the piston rings fit well.
  • This improves compress
  • Also if air seeps past the rings it burns in the cylinders producing black smoke(soot/carbon)
  • Poorly fitting rings results in an inefficient engine

Differences between Diesel and Petrol engines

Diesel EnginePetrol Engine
Diesel fuel is usedPetrol is used as a fuel
Fuel is injected by the fuel injector after the air has been compressedPetrol/Air mixture is introduced into the cylinder through jets
Fuel is ignited by the heat of compressed airFuel is ignited by sparks from spark plugs
High compressionThe compression is half that of the diesel engine
The engines are heavierThe engines are lighter than diesel engines
40% with less carbon monoxide are thus more economical25% efficient more carbon monoxide thus less efficient

To access more topics go to the Combined Science Notes page.