Help with learning why diesels don't have vacuum?

[SZR48207]

I am having a very difficult time understanding why exactly diesel motors (generally) don't produce a vacuum like a gasoline engine does. Since both are operating on the 4 stroke principle, I don't see how the movement of a piston downward in a cylinder in a diesel motor isn't drawing in air, thus creating a vacuum.

I have scoured Google and here is what I have read so far:

1. Diesel engines use fuel injection to control the throttle, and this is also why they don't have a throttle

2. In a way, a diesel engine runs constantly at wide-open-throttle.


Where I'm perplexed is how a diesel engine isn't constantly at max RPM if it runs in the manner described above.

 

[Shannow]

Power comes from the stored energy in the fuel.

In a diesel, the amount of fuel injected controls the power output, and when it's a small number, the power output is small, regardless of how much air it is pumping through.

In a gasser, it's the same, but a spark ignition engine needs a certain air/fuel ratio to even run, so to control load you need to control the air that goes in as well.

 

[KingCake]

https://www.youtube.com/watch?v=rXVJG9n6BAs

They don't run at max rpm b/c the injectors are only putting in so much diesel. Now a diesel will run away if it gets unlimited fuel, like if somehow oil or diesel start leaking into the engine.

 

[shDK]

Diesels Don,t have a throttle-body. Like a petrol car has. The throttle on a diesel is actually more of a air restriktiv. And most of the time it is full open. But when it is acticve it Is mostly during cold start, or when shutting of the engine. When shutting the throttle at turn off. The engine make wacume in the intake and the engine stops more soft.

And most diesels today have turbo chargers. So There vil allways be positive intake pressure.

 

[SteveSRT8]

A regular gasoline engine is ALWAYS trying to go full speed. That's why we must "throttle" it in order to slow it down. Control the air and you control the engine's speed.

Diesels use tight control over the fuel to do the same thing. Both the timing of the injectors and the quantity of fuel are tightly controlled in order to control speed.

Since most diesels do not have throttle plates in the traditional sense they can be more efficient in some applications, too.

 

[mjoekingz28]

Remember 'dieseling' from Hollywood movies? I wonder if it was common back in the day......like the 1970s?

I wonder if whatever caused a car to diesel when you turned the key to off is a similar occurance to what you are asking with regards to vacuum!

 

[Colt45ws]

Yup, look at videos on YouTube of diesel runaways. Youll see what happens when they are given unlimited fuel. They try to run at maximum RPM and quickly destroy themselves unless someone blocks the air intakes. I wanted to link one in particular of a old Detroit 2-Stroke in a barn that an old man and a couple of young guys are trying to get started. Well, it starts, but those Detroits are somewhat infamous for having the fuel rack (which controls the injection of fuel) sticking at full-throttle after they sit for awhile. So, it took off, screaming at maximum RPM.
The old guy stuck a 2x4 over the air intake and that brought the engine down to a dull roar and finally he stalled it out by holding a rag over the intake as well.

Diesels run really really lean at idle. Even if not really accurate, it helped me to think of gas engines as always having the typical little 'explosions' where most of the chamber volume is consumed each time.
Diesels, on the other hand I would imagine as the injector kind of being like a flame thrower in the chamber.

 

[Jetronic]

yes, the flame thrower analogy is quite accurate... like an oil furnace....

 

[rustypigeon]

It has been pretty much covered above, but I will throw in my 2 cents.

The vacuum in a gasoline engine is created from the downward motion of the piston sucking against the throttle plate. The more you open the throttle plate, the less vacuum there is. When you are going down hill in a gasoline car and let off the gas, the vacuum helps slow you down. The piston is trying to suck air in through the restriction, and this resistance helps slow the car down.

Since a diesel does not have a throttle plate, there is no vacuum. Engine speed in entirely controlled by the amount of fuel injected, as stated in the above posts. When you are going downhill in a tractor-trailer there is no vacuum created when you let off the accelerator pedal. The vehicle will continue gaining speed even when the accelerator is let go. This is why most tractor-trailers have a "jake brake". The jake brake, when activated, closes off the intake valves. This creates vacuum inside the combustion chambers and helps keep the speed under control when going downhill.

 

[bullwinkle]

There's normally no restriction at all in the intake, it runs with all available air at all times, the fuel is the only thing that is regulated (on a non-turbo diesel). For crankcase vapors, there can be a fitting that protrudes into the airflow that can pull blowby gases in (CDR valve or similar), but there's not a lot of vacuum there.

 

[The_Jeff]

My merc 240d had some sort of half baked throttle valve that seemed to exist solely to pull a little vacuum at lower loads for its crude crankcase vent system. The "throttle plate" fit very loosely and was mechanically connected to the rest of the Rube Goldberg accellerator pedal linkage.

 

[SwampSurvivor]

Originally Posted By: mjoekingz28
Remember 'dieseling' from Hollywood movies? I wonder if it was common back in the day......like the 1970s?

I wonder if whatever caused a car to diesel when you turned the key to off is a similar occurance to what you are asking with regards to vacuum!


Dieseling in a gas engine is a bit different. The throttle is still closed. But because the engine was so hot, for whatever reason, the fuel still dripping in from the carb was enough for it to ignite from heat and compression. Basically pinging.

Fuel injection engines don't do it because when you turn the key off, you stop both fuel and spark.

https://www.youtube.com/watch?v=wo8cVCTvv-I

The engine is off, but while it was still rotating after the key was turned to off, fuel was still getting sucked in and it was igniting because the engine is hot

 

[mahansm]

Gasoline engines are ignited by a spark. The fuel/air mix must be within a certain ratio in order for ignition to occur. To regulate power output of the engine, the amount of fuel/air mix in the cylinder must be regulated; therefore a throttle and at less than full output, vacuum in the intake manifold.

Diesel is compression ignition; however much fuel is injected near TDC on compression stroke ignites and burns (up to a certain limit). The hotter the air in the chamber before injection, the better the ignition. More air, more compression, higher temperature, better ignition. Power output is determined by the amount of fuel injected; more fuel, more pressure, more torque. Since there is no need to limit the amount of air in the cylinder and, in fact, limiting the amount of air will hurt engine performance and efficiency, there is no throttling and no manifold vacuum.

Years ago I did some temperature calculations for my 300D (OM617.952) Mercedes. At 0 degrees F inlet temperature, at idle, cylinder temp at TDC before injection was about 500 F. On a 100 F day at full boost, TDC temp before injection was closer to 1400 F. This assumes adiabatic compression and 70% efficiency through the turbocharger.

 

[SZR48207]

Thanks guys. The flamethrower analogy seems to help make sense of it.

 

[abycat]

diesels don't need a certain air fuel ratio like gas engines. That also contributes to their efficiency. Some do have a plate like mine but it's for making vacuum so the egr can pull exhaust gasses. I did remove mine though since the egr system in my truck was removed.

 

[Chris142]

There's no throttle blade.that's what makes vacuum in a gasoline engine.the diesel intake is just a wide open hole and rpm is controlled by the amount of fuel injected