Fuel Direct
How fuel injection in a gasoline engine is different from direct injection in diesel engines?
…I’m a mechanical engineering student…so technical terms are welcome..
If possible could you please tell me the difference between the injectors used ?
The basic difference between fuel injection for gasoline and direct injection for a diesel lies in the position of the injectors. The gasoline injector is placed in the intake manifold so that the fuel squirts under pressure into the runner of the intake manifold to mix with air drawn in during the intake stroke of the piston (basic four stroke cycle: Intake, Compression, Ignition and Exhaust). The air/fuel mixture is drawn into the cylinder much like after a carburetor mixes fuel and air together in a conventional carburated engine. There are even engines which use a throttle body style carburetor which has one or more injectors on top. After the compression stroke a spark from the spark plug fires the air/fuel mixture and the piston is driven down which provides power and then the exhaust stroke takes place which completes the cycle.
In the direct injection Diesel Engine the injector tip is actual placed in the combustion chamber. this prevents the mixing of the air with the Diesel Fuel. The rest of the cycle is similar to the gasoline engine except there is no need for a spark to ignite the fuel because diesel engines have a higher compression ratio (Generally gasoline compression ratios are between 8:1 to 10:1 where as diesel engines are between 14:1 to 17:1) and will ignite on their own. Most diesels also harvest the wasted exhaust pressure as it escapes the combustion chamber and passes it through a turbo charger. The turbo charger has a turbine which the exhaust fumes can spin (up to about 20,000 rpm.). This turbine is attached to another turbine which forces air into the engine’s cylinders via the intake manifold under pressure (Known as the Turbo Boost Pressure, usually between 10 PSI and 13 PSI I think.).
With the increased air pressure from the higher Turbo Boost and the increased compression ratio it is only common sense that the diesel injectors have to operate under higher pressures than gasoline fuel injectors. Diesel injectors have come in different styles over the years if I remember right. There were some that were completely mechanical in nature (I think they were basically just a small pipe for the fuel but there was a rotary valve which directed the flow to different lines for different cylinders) and today’s diesels generally come with electro-mechanical injectors to be used with electronic engine system just like gasoline systems. As to more differences between the two you may need to look to an engineer to explain them.
You can copy and paste the following into you browser for a detailed look at the differences between gasoline and diesel engines auto.howstuffworks.com/diesel2.htm
PS: I understand that a Japanese engine manufacture has patented a direct liquid injection system for propane engines used in the forklift industry. While you might not find this very interesting, think about this: Gasoline has a rated octane level between 87 and 92, Natural Gas peak rating is 112 (but averages below 98 octane) but Propane has a rated constant octane level of 104. Propane also loves a high compression engine.
Just fuel for some racer’s mind to think about………..
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