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Name: Peter
Status: Other
Grade: Other
Location: Outside U.S.
Country: England
Date: January 2008


Question:
I can understand that to operate efficiently, a diesel engine requires a higher compression ratio than a petrol engine and as a result the diesel engine needs to be made from stronger materials to reduce breakdowns. But, can a diesel engine be mass produced more cheaply than a petrol engine!



Replies:
Hi Peter,

No, a diesel engine can never be made more cheaply than a gasoline (petrol) engine. The addition strength needed is only a minor contributor to cost. The main thing that makes a diesel motor more expensive is the requirement for a high pressure (greater than 20,000 psi on modern engines!) direct cylinder fuel injection system. This is far more expensive than even the most sophisticated gasoline injection systems. A diesel injection system requires this super high pressure, ultra-precision fuel pump, whereas a gasoline engine's injection system's pump is a relatively simple electric pump that needs to develop only 30 to 90 psi. It is mainly the fuel system that makes the diesel engine so much more costly, and this is driven by the requirement that the fuel in a diesel engine must be injected directly into the combustion chamber, whereas a gasoline engine can operate very well with fuel simply being injected into the inlet manifold, right behind the inlet valves. So called "direct injection" (into the combustion chamber) requires far higher pressures than so-called "port" (manifold) injection, and high pressure pumps are highly expensive, and high precision components.

Regards,

Bob Wilson


A diesel engine uses a higher compression ratio not only for thermodynamic efficiency, but also so to achieve compression ignition, that is, the air in the cylinder gets hot enough when compressed to ignite the injected diesel fuel.

It does not seem possible to manufacture a diesel engine for lower cost than gasoline engine because the diesel must be more rugged and also requires a much more costly high-pressure fuel injection system (tens of thousands of psi). A spark-ignited gasoline engine uses a comparatively simple low-pressure fuel injection system and milder compression ratio.

Robert Erck



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