Diesel
Practical Action
Secondly, there is the sub-division according to cycle type; the two stroke or four stroke cycle.
This means that engines have an ignition phase on every revolution of the crankshaft or every
other revolution.
The method for mixing and injecting air and fuel is different for the two cycle types. Most IC
engines use the four stroke cycle.
The four stroke cycle - diesel engine
1st stroke:
2nd stroke:
3rd stroke:
4th stroke:
induction stroke - while the inlet valve is open the descending piston draws in
fresh air.
compression stroke - while the valves are closed the air is compressed to a
pressure of up to 25 bar.
ignition and power stroke - fuel is injected while the valves are closed (fuel
injection actually starts at the end of the previous stroke), the fuel ignites
spontaneously and the piston is forced downwards by the combustion gases.
exhaust stroke - the exhaust valve is open and the rising piston discharges the
spent gases from the cylinder.
Cylinder
PPisistotonn
Induction
Compression
Ignition
Exhaust
Figure 2: The four-stroke Diesel cycle
Since power is developed during only one stroke, the single cylinder four-stroke engine is not
uniform in its power supply. Smoother running is obtained with multi cylinder engines because
the cranks are staggered on the crankshaft. There are many variations of engine configuration
e.g. 4 or 6 cylinder, in-line, horizontally opposed, vee or radial configurations.
The Table below shows the relative practical advantages and disadvantages of petrol and diesel
engines.
Diesel
Pros
Lower fuel costs
Higher efficiencies
Readily available for a wide range of sizes
and applications
Lower running speeds
Cons
Maintenance is more expensive
Heavier and bulkier for a given power
Higher capital cost
Pollution
Petrol
Pros
Light - hence more portable
Lower capital costs
Cheaper to maintain
Higher running speeds
Cons
Not so durable - especially under continuous
long-term usage
Lower efficiency for equivalent power
Fuel more expensive
Narrower range of off-the-shelf engines
available - smaller engines more readily
available
Pollution
2