How diesel engines control air flow

[rt54321]

Greetings all!

A question for you, regarding how diesel engines work.

–> With a gasoline engine, if I press the accelerator suddenly, the throttle valve opens more, allowing more air to get to the engine
–> Using the MAF/MAP sensors, and the O2 sensor, the car computer then calculates the proper amount of fuel for the increased amount of air to get back to the stoichiometric ratio of 14.7:1.

In a diesel, if I press the gas pedal, then more fuel is added to the engine, causing it to go faster. Diesels (most of them anyways) do not have a throttle valve to control air flow.

–> Is this to say that, diesel engines do not need to be at stoichiometric ratio of 14.7:1? If my statement is wrong, how does the engine meter/control the right amount of air, for the given amount of fuel?

[Please correct me, if any of my assumptions stated above are wrong]

Thank you!
-R

Topic

[Jim]

Imagine a gasoline engine without a butterfly valve, and where the fuel mixture is controlled directly by the gas pedal instead of calculated from air flow and temperature. A naturally aspirated diesel engine will suck in what it can, and a turbo diesel will push in what it can. A lean condition in a diesel will mostly only cause it to lose torque, while a rich mixture will cause excessive heat and soot, unlike in a gasoline engine where these conditions can be catastrophic. I bet there would be no problems designing a diesel engine that uses a butterfly valve and some electronics to control the ratio (and for all I know, more modern diesel than my own are doing it this way).

[dan]

Lets review the differences between a gasoline and diesel engine, and there are few differences!

Gasonline engines
.Run on Gasoline
.Use spark plugs for ignition
.compression ratios between 8-1 10-1 for average cars 11-1 15-1 high performance
.engine speed controlled buy air and fuel flow
.fuel injected during intake stroke or drawn through carburetor

Diesel engines
.Runs on Diesel fuel, or fuels similar
.Uses hot gas from high compression to ignite air fuel mixture
.compression ratios up to or exceeding 25-1
.engine speeds purley controlled by fuel
.fuel injected during power stroke

With a diesel engine the biggest differences are the higher compression which produces much higher combustion chamber pressures, therefore you get much higher combustion chamber temperatures, temperatures in a gasoline engine that would cause spark knock MANY TIMES before the piston would reach top dead center. another big difference is there is no spark plugs in a diesel, usually…. unless its a malty fuel diesel engine. (different subject.) the fuel is injected during the compression stroke just before the power stroke, when the diesel fuel is injected into the hot compressed air it ignites, expanding pressing down the piston.

with a gasoline engine since the fuel is injected with the incoming air it is essential that air flow is controlled, but with a diesel engine the fuel is injected during the power stroke, and therefore it is not so essential airflow is controled, in fact with a diesel engine you want as much air flow as you can get at times, because the more compression you have the hotter the combustion chamber temps are, the better it burns that fuel when its injected!

[Antoni]

[quote=”rt54321″ post=149881]Diesels (most of them anyways) do not have a throttle valve to control air flow.[/quote]

This throttle valve is only used to shut down the engine slowly, without big engine vibrations (Like older engines).

You can run the engine without this device. [hr]
[quote=”rt54321″ post=149881]…diesel engines do not need to be at stoichiometric ratio of 14.7:1?[/quote]

Of course it needs, every engine need this. (The stoechiometric ratio is 14.5:1, by the way.)[hr]
[quote=”rt54321″ post=149881]…how does the engine meter/control the right amount of air, for the given amount of fuel?[/quote]

Diesel engine load works only with quantity.

The engine controller knows exactly the right amount of air by the displacement of one cylinder (programmed in the factory). So the engine computer give the right amount of fuel to the cylinder.

[Author]

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