I've been on the site here for several months, even done a few minor bombs on my truck, but in my brain (scary place) I can't seem to grasp two things in the diesel world (and they may even be related)?

Why do "most" use a turbo?

A diesel is a 4-stroke motor, why no vacuum (would they have a vacuum if no turbo? Is it because there is no throttle plate?)???

I've hit several websites, learned a lot about how a diesel functions, and besides the no spark plug thing and high compression that ignites the mixture, they don't look much different than a 4-stroke gas engine to me.

 

I will try. gasser use a restricted air system, carb butterfly/tb, to regulate the air charge to the engine. when the buterfly is closed or part open the cylinder(s)(displacement) is looking for more air than is availible and pulles a vacume. If you ever noticed when you stomped on your throttle for an extended time the vacume operated stuff quits, thats why they put reserve canisters on them,. This is because the butterfly is wide open and the engine has all the air it demands and no vacume in intake manifold. The diesel uses free air delivery in that the cylinders can draw full displacement with no restriction so no or little vacume. this is why we worry about air filters, to small or dirty is restriction and the fuel charge goes rich and we smoke and lose power. turbo's pack more air into cylinders to allow more fuel to be used.
Goc, I can think it, can't type it.
If you were not confused you are now
Clark

 

I can give you a real life example of why most diesels are Turbocharged.
You are welcome to test drive a 4600 series International (7.3L) Box Truck I use for one of my businesses.
The thing can't get out of it's own way. She boasts a whopping top speed on 63MPH on flat ground, closer to 55 on ANY grade empty. If she's loaded,........... you will be the subject of many fancy hand gestures on I-95.

 

Okay and thanks folks. I understand the vacuum and it was as I expected, the absence of a throttle plate (restriction).

The turbo, I know there are several non-turbo motors out there, and yes PJ I know most are serious dogs without a turbo. But why turbo? Is it because it is more dynamic than a supercharger, more fuel, more boost? Is adding displacement too difficult to get the torque required that a turbo offers?

Sorry if I'm getting too deep into diesel theory here gents, just trying to understand the whole enchilada.

 

Mmmm, wasn't it Gale Banks who was the first person to outfit a diesel engine with a turbo?
I would say since the diesel moves a massive amount of air, a supercharger (running off of a belt) wouldn't move the volume??

Just a guess on that one.

 

A turbo charger is more efficient than a supercharger. The turbo takes waste heat that would otherwise just go out the tailpipe and converts it into boost. A supercharger takes shaft horsepower to turn the compressor. Nuff said.

Edwin

 

Okay I'm cool with turbo vs supercharger, why not displacement? Why do "most" diesels have a turbo vs most gassers don't?

In a gasser - more air equals more power, bigger carb, higher longer duration cam to squeeze more into the cylinder, then just match the gas to the amount of air.

Sorry if I'm sounding on this...

 

''Why do "most" diesels have a turbo vs most gassers don't?''

I'll take a swing here...it's not so much the difference between gas and diesel as it is between spark ignition and compression ignition. In the case of a spark ignition gasoline engine, you're always looking for the stochiometric ratio of 14.7parts air to 1 part fuel..no matter what the load or rpm. Adding a turbo to this mix makes it very easy to run lean, which will cause detonation, piston damage, etc. I'm guessing its just plain easier to punch out some more cubes for more power. With a diesel operating on compression ignition, there's no limit to how lean you can run...nor how rich, depending on your tolerance for smoke, hehe. The other part of my guess is that since diesels find themselves primarily in work vehicles, weight is always a concern. If Cummins offers me 160hp/400tq at 800 pounds with a turbo-charged setup VS 160/400 at 1500 pounds in a naturally aspirated cube-monster, it's clear to see the GVW will work out in my favor with the little turbo engine. just a couple guesses

 

Good post^^

The turbo is not only more efficient, it's very easy to integrate in a a variable-ratio diesel.

Turbo charging a gasser brings more complex engine management issues, and detonation is a big concern.

The match of a turbo and a diesel is a "match made in heaven." The complement each other perfectly.

 

Turbos, by far are the more efficient way of producing boost pressure compared to a crank driven supercharger. But there is no reason why a supercharger can't easily pump the same volume if not more than the turbo - it will just require more hp to do so. My favourite example of the higher efficiency of turbocharging is the supercharged Mustang Cobra......when the factory blower is removed and is replaced with a turbocharger, the engine picks up an additional 150 hp. Also, the boost psi to hp ratio increase is not comparable between the blower and the turbo....the turbo produces proportionally more hp per psi of boost than the blower and this application is a perfect example of that.

Most spark ignition engine applications would typically be far too powerful with the addition of a turbo charger. For the most part, they can meet their power requirements without the addition of a turbo.

The diesel benefits greatly from a turbo for a few reasons but the most obvious is compression. Since the diesel is a compression ignition engine, it relies soley on the rate and amount of compression to ignite the air/fuel charge. More cylinder pressure allows the cylinder to accept more fuel which in turn will make more power. Since the turbocharged diesel does not need anywhere near as high of a static compression ratio to light and fully burn the intake charge, it can be more mechanically efficient. The higher the compression, the bigger and more heavy duty the engine parts need to be to prevent them from wearing out or failing because of the CONSTANTLY higher loads that are put on the components. But think of the turbo charger as a way of producing dynamic compression. The engine can now run at much lower static compression ratios but still be capable of much higher cylinder pressures AND have the cylinder pressure be variable with turbo boost pressure to accomadate any level of fueling. When fueling and the power requirement is low, cylinder pressure is low but when the power requirement is high, the cylinder pressure can be increased accordingly. The addition of turbo really allows for a much smaller and more efficient diesel engine to pump out more power than engines that are much larger.

The reason most diesel engines don't produce vacuum is because they are unthrottled engines....meaning, regardless of the fueling level, they are always ingesting a full air charge. Instead, throttling is done by altering the air / fuel ratio with the injector pump. There is no vacuum present because there are no throttle plates that close to create vacuum. A constant air / fuel ratio does not need to be maintained like a spark ignition engine.

 

One other thing to consider is engine RPM.
A turbodiesel builds it's power at normal driving RPM.
The gassers have to run at twice and some even at 3 times the RPM of a turbodiesel to build the same power. The gassers use alot of fuel too. In order to get the good advertised power, a gasser has to be cammed up for high RPM to achieve the good advertised power. The trouble with that is the extra cam timing hurts normal driving RPM power and fuel economy, but you look good on paper with 200HP at 5800 rpm. 200HP at 1800 usable RPM is easy for a Cummins turbodiesel.

 

Clunk basically hit on this.

I'll break in down even simpler yet.

Diesel engines by their very nature produce their power VERY LOW in the RPM band and have a VERY VERY NARROW torque/horsepower range at that. That is one of the main reasons diesels are good for towing and hauling heavy items.

NON-Turbo'd diesel vehicles are VERY slow to come up to speed and can barely get out of there own way.

I used to own a 83 Fullzize Blazer with a 6.2 diesel and it was scary SLOW. I also used to have a 82 VW Rabbit diesel. The only thing slower than it was a U-Haul truck!

Gassers produce more "USEABLE" horsepower throughout a wider RPM range WITHOUT using a Turbocharger.

Now through a Turbo on a diesel and LOOK OUT!

 

We will start to see gas engines without throttle plates soon as well. No vacum in the engine means less drag on the intake stroke. There will be no cams or valve train, timing chains or anything. All the valves will be computer actuated hydraulic/electric solenoids. Infinite Valve timing adjustment for any operating conditions.

 

Allright!! Chip up a big cam for saturday night cruises at the Burger Doodle!

 

quite possibly one of the best threads ive read on here for info on how a diesel works.


thanks for the great info guys!