In my readings I have learned that a N/A diesel engine always intakes the same amount of air, and furthur, this amount is always more than it 'needs' in relation to the amount fuel injected with respect to combustion.

So in a nutshell, how does the turbo (packing even more air into the cylinder) boost HP etc.

I'm not stupid, I realize the short answer might be that the TD injects more fuel, but TD owners MPG claims don't seem to support that. Any other factors at work here??


Thanks for reading this, I just can't say enuff good things about this forum and its members.

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Cheers,
Robert

I'm sure others more knowledgable than I will respond, but in a nutshell you have it correct. A turbo-charged engine just injects a little more fuel. The mpg differences would come probably because the fuel is combusted a little more completely than a N/A engine. The amount of extra fuel injected under high boost conditions is controlled by the ALDA-in noncomputerized engines.

As an aside to this discussion, I have often wondered how much a role the simple presence of the added air makes. For instance, a jet engine works only on the expansion of hot air. The fuel is only there to create heat to expand the air. I have never really found an answer to what extent an IC engine works on the same principle. Since a turbo-charged engine is cramming more air into the cylinder, that is more air that can undergo expansion and exert force on the piston. Physical chemistry was too many years ago...

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Keep everything as simple as possible-but no simpler--Albert Einstein

Due to volumetric inefficiencies NO n/a engine will ever achieve 100%. Therefore you are limited in how much fuel you can inject without melting something..

With a Turbo you get greater than 100% volumetric efficiency which lets you make more power when its needed most ((via more fuel) ...so same power as a larger engine, while at the same time when the extra power is not needed you run as the much smaller and efficient engine that it is. Its in effect a more efficient engine, and a more powerful engine...at the same time.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

You have it correct. The thermodynamics of an internal combustion engine are fairly straght forward. 1) Cool, low pressure air is admitted into the combustion chamber, 2) the air is compressed by the piston (which requires energy), 3) energy (pressure) is added to the air by combustion of a fuel source and the O2 in the air, 4) the piston is driven down by the increased pressure in the combudtion chamber (providing more energy than was required to originally compress the air). In both a gas and diesel engine, additional power can be obtained by increasing the amount of air in the cylinder with a turbo charger. This is a way of making an engine "act like it has more displacement" by having more air available during each pistion stroke.

From a thermodynamics point of view, the difference between a gas engine and a diesel engine has a lot to do with the speed of combustion within the cylinder. To oversimplify, a gas engine has very fast combustion that occurs when the pistion is near the top of its stroke (causing a pressure "spike"). This pressure pushes the piston down a lot at first, and less as the pressure reduces. In a diesel engine, the combustion process is slower and provides less of a "spike" at first. Instead the combustion process continues as the pistion is pushed down at a more constant pressure.

To finally try to answer you question, both a N/A and a turbo engine will use as much fuel as required to provide the required power. A turbo engine only uses "extra" fuel when the driver uses extra power. Each pound of fuel contains a specific amount of available energy. If it takes a given amount of energy to get your car from point A to B, that determines how much fuel is required. However, internal combustion engines are not very efficient, and most of this energy is wasted. In the real world, fuel consumption is controled my the efficiency of the engine, including how it is operated.

Everyone asleep yet?

in any internal combustion engine.

In a N/A engine, air at near atmospheric pressure (and volume), has fuel injected to make hot air which expands at a certain pressure.

With a turbo engine, air at a higher pressure (same volume), has a little more fuel injected to make hot air expand with a higher pressure (quicker).

The engine is doing the same amount of work, moving the same parts and weight of the car, so the mileage (efficiency) will not vary that much. If you think of air as a liquid, the physics is a little easier to grasp. (volume + pressure)

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Sam

84 300SD 350K+ miles ( Blue Belle )

In a Diesel engine, the highly compressed air also contributes it's own energy back into the upcoming power stroke. When you add the turbo, this provides even more air to accomplish this. Also, some Turbo Diesels are built with lower compression ratios than their NA counterparts (not MB I don't think), so the turbo, which is re-cycling free energy from the exhaust by "pre-compressing" the air charge has already done some of the compression work that would have otherwise been done within the engine. Naturally, the lowered compression is not lowered so much that it would make it hard to start- just enough to allow for the turbo building that extra boost externally.

Dave

'76 300D

but in the end to make more power you use more fuel every time the engine rotates. the way that the mileage is kept reasonable is by the

higher gear ratio

in the axle that allows the engine to turn less rpm at a given speed. the same holds true if a normally asperated engine is bigger and more powerful, you can achieve the same mpg with the same car by using higher gear in the rear end or an overdrive in the tranny.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Hi Craig,

Does the constant pressure of combustion in a diesel engine contribute to it having more torque as compared to a similarly-displaced gas engine?

This thread is interesting, thanks for bringing it up.

Hi,

The short answer is yes. There are lots of factors in the design details of a diesel engine, but in general diesels have their maximum torque at a lower rpm than a similar gas engine. This is related to the fact that the combustion process in a diesel is slower and tends to provide a more constant pressure on the piston during the "power" stroke. If you operate a diesel at higher speeds, the pistions move faster and tend to "get ahead" of the expansion of the gases due to combustion. This results in a reduction of torque at higher rpms.

In a gas engine, with much faster combustion, the torque tends to increase at higher rpms where the speed of the pistion and the speed of the expanding gas is "better matched".

Having said all that, modern diesel engines do not really operate with constant pressure during the entire combustion process. In the interest of efficiency, modern diesels have the combustion process begin prior to the start of the "power stroke" resulting in an increase of pressure prior to the start of the stroke, followed by a (more or less) constant pressure expansion process. But you are correct, this design results in more usable torque at lower rpms.

It clarify a lot of the Q. in my small head. Explains well why Dsl has lots more torque than similar gas engine.
My Turbo just wind up real well when going up hill!
Gas engine I never saw something has as much power.
Plus if i drove a gas engine like I drove my Dsl, I need to pull a bank job every week inorder to support my lead foot habit!

responds to engine heat, so when you begin to pull a hill the heat rises, the turbo spools up and soon the cars speed will stabilize wo pushing the pedal down anymore. after driving turbo diesels for almost 20 years it still fascinates me.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I am told that turocharging improves both fuel economy and the horsepower at the same time in diesel engines, in gas engines turbocharging gives you either fuel economy or more horsepower but not both, Is this true?

Vahe
240D 77/350K

what i have read too. although in practice i could not verify it. in reality the non turbo cars that i have had get better mileage thanthe turbo but that proves nothing since it all depends on the gear ratios. imho i think that prob any difference in economy is small.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Excellent thread... One of the best in a long time?

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#dieselFLEET
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'97 E300
'99 E300

No,
Turbocharging gives more power from the engine available to the driver.

a. If the driver uses all of that power, he will use more fue to generate that power.

b. If the driver only uses the power when needed, he may use slightly less fuel because he has the pedal to the metal for a much shorter time, even though the engine is using more fuel while the power is applied.

c. The faster a car travels, the more power it requires to maintain that speed. Wind resistance increases as the square of the velocity.

The real life situation the economy may improve slightly in a turbocharged car, provided that the driver does not travel faster than before, or race between traffic lights.

Slowing down by 10 mph can make a significant reduction in fuel consumption.

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Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.