The main difference between Direct and Indirect Injection is the layout of the injection system, the Indirect Injection System actually has a small swirl chamber above the cylinder, where the fuel is injected, this chamber also contains the glow plug, which is much needed to start the engine, the Direct Injection system has the injection nozzle actually fixed to the top of the combustion chamber, usually the piston on the engine with this type of injection system has a crown shape in the top to create the needed swirl, the diagrams below show the difference between both systems.
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The Advantages of an ‘Indirect Injection’ system compared to ‘Direct Injection’ are:
- High rate of swirl over wide range of engine speeds
- Does not require expensive, ultra high pressure injection system
- Less chance of injector blockage due to self cleaning pintle injectors
The Disadvantages of an ‘Indirect Injection’ System compared to ‘Direct Injection’ are:
- Poorer fuel consumption due to lower thermal efficiency
- Higher compression ratio required to aid starting
The Advantages of a ‘Direct Injection’ System compared to an ‘Indirect Injection’ System are:
- Cold starting is easier
- More Economical
- Smaller combustion space, better thermal efficiency
The Disadvantages of a ‘Direct Injection’ System compared to an ‘Indirect Injection’ system are:
- Tend to be noisy
- More prone to blockages due to small injector holes
- Lower power output
- Slower swirling, particularly at low engine speeds
Direct and Indirect Injection Cycle
Induction, the descending piston increases the cylinder volume and decreases the pressure. Atmospheric pressure forces air through the open inlet port in the cylinder.
Compression, both valves are closed and so the ascending piston compresses and raises the temperature of the air.
Power, just before TDC (Top Dead Centre), fuel oil, having an ignition temperature of 400 degrees Celsius, is injected into the cylinder at a high pressure. After a short delay, the fuel begins to burn and liberates heat which raises the pressure, providing the thrust necessary for the power stroke.
Exhaust, as the piston nears the end of the power stroke, the exhaust port is opened. The ascending piston pumps out the burnt gas in readiness for the new cycle.
Direct Injection Cycle
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Indirect Injection Cycle
Image Copyright © Nicholas Seal. Do not copy.
