On a gasser, you need a throttle body to rev the engine. Otherwise, it would just draw insane amounts of air and cause the RPM's to sky rocket. Why don't you need one on a diesel?

 

Because on a diesel engine you control the amount of fuel not the amount of air.

 

Here's the best explanation in laymans terms...

http://auto.howstuffworks.com/diesel.htm

 

You also have to understand that you really can't burn diesel in a too lean condition. The gassers have a real problem with trying to burn lean. They work on the opposite principal of a diesel engine. The more gasoline you put in the cylinders the cooler the temps will be. The more diesel you put into the cylinders the hotter the temps will be. The gassers also need to limit the amount of air drawn into the engine. The throttle plate is what causes the engines to produce a vacuum also. The gassers also need a fairly precise fuel to air ratio to run properly.
Tom

 

Hmmm. I get it now. Thanks for the help yall.

 

The Throttle doesn't exactly cause engine vacuum. The pistons do. The Camshaft controls the amount of vacuum also, by determining how long the valves stay open, overlap, etc., just like our diesels. The longer the stroke, the longer air is drawn in, but displacement is really the key to how much air. Some gas engines need to add a seperate vacuum reservoir since the cam is too "large" to allow good vacuum at low throttle or off idle running RPM's, preventing such things as power brakes from working correctly.

The Intake Runner dimensions and the throttle body or carburetor determine how well the vacuum is regulated for efficient mixing of fuel and air. Unlike our compression engines, liquid gasoline, though very flammable, does not fire very well unless completely vaporized. Therefore, precisely metered amounts of air and fuel are mixed together throughout the carburetor operating range to provide the proper amount of flammable vapors to ignite and create power. Fuel injected vehicles use a TPS and computer control to relate the amount of gasoline to inject relative to the aircharge going through the throttle plate, similar to our 24 valve injection system. The Computer uses the MAP sensor to determine the charge relative to ambient air pressure (and sometimes temperature, depending on the system) and throttle position. Instead of relying entirely on engine vacuum to determine the fuel needs like a carburetor, fuel injection has the potential to provide just the right amount of fuel all the time, which translates to greater efficiency.

Sorry for the long winded explanation, but thats it in a quick nutshell. Another difference is the rate of burn is generally much faster in a gas engine than a diesel due to fuel characteristics. Our Cummins burns almost completely down the stroke, pushing most of the way creating great torque at low RPMs, while a gas engine is more of a controlled explosion, burning short and fast, relying on inertia and rotational speed to make power, therefore moving the power and torque curves to higher RPMs. Each has its purpose and are useful in different applications, depending on desired results. Just check the tach on a heavy hauling gas engine and see where the best throttle position is for towing compared to the diesel engine.

My $0.02

 

Josh thanks for the more complete explanation. I was trying to keep it somewhat simple

 

No sweat!