the relationship between diesel engines, turbochargers, and efficiency

[Skygrasper550]

I keep seeing these 3 words together in car dealership brochures, adverts, and info on diesel-powered cars in general. I've read some info about them and tried my best (really!) to understand them but to no avail. So, to cut the story short, how does a turbocharger make a diesel engine more efficient?

I mean, the only advantage I can think of is the turbo sort of "balances" the diesel engine's powerband. While a diesel engine does provide enormous performance at low RPM, it sucks completely at high RPM. So the remedy for that is a turbocharger, which then provided high RPM power when needed. But other than that, what other advantages does a turbocharger provide to a diesel engine to make it more "efficient"?

 

[Beeblebrox237]

https://en.wikipedia.org/wiki/Diesel_engine#Supercharging_and_turbocharging

https://en.wikipedia.org/wiki/Diese...d_disadvantages_versus_spark-ignition_engines

Took me 60 seconds to find those two very relevant links. Is it really that hard to google something?

 

[Skygrasper550]

https://en.wikipedia.org/wiki/Diesel_engine#Supercharging_and_turbocharging

https://en.wikipedia.org/wiki/Diese...d_disadvantages_versus_spark-ignition_engines

Took me 60 seconds to find those two very relevant links. Is it really that hard to google something?

Those links are what I meant by "some info", along with the "Turbo-diesel" article. I've read them yes, but I can't quite understand them.

 

[jimipitbull]

Diesel + air + heat = chug

Diesel + compressed air + heat = CHUG

Higher revs = turbo spins faster = more compressed air = louder faster CHUGS


Anyone know if there is a reason why no one seems to supercharge diesel engines instead of the turbo?

 

[bremics]

Diesel + air + heat = chug

Diesel + compressed air + heat = CHUG

Higher revs = turbo spins faster = more compressed air = louder faster CHUGS


Anyone know if there is a reason why no one seems to supercharge diesel engines instead of the turbo?

Because superchargers are more expensive/complicated?

 

[jimipitbull]

Because superchargers are more expensive/complicated?

No, they are much the same as a turbo, in that they both compress air. Supercharger spins off a belt, turbo spins from exhaust pressure.

 

[Sage Ages]

So, to cut the story short, how does a turbocharger make a diesel engine more efficient?

Typically, a turbocharger may be installed on a smaller, more efficient engine to allow it to retain the same perfomance as a larger displacement naturally aspirated engine. I'm not too familiar with diesels, but I'm pretty sure the same thing applies here.

 

[Vegard]

Anyone know if there is a reason why no one seems to supercharge diesel engines instead of the turbo?

Mazda did make a supercharged Diesel in the 90's.

I think the combination of the parasitic effect (Diesels are already highly stressed with their high compression ratios) and the boost characteristics of the efficient supercharger types makes it less desirable than the super efficient turbocharger.

 

[bremics]

No, they are much the same as a turbo, in that they both compress air. Supercharger spins off a belt, turbo spins from exhaust pressure.

I'm pretty sure superchargers are more expensive and difficult to install at least.

 

[Sage Ages]

I'm pretty sure superchargers are more expensive and difficult to install at least.

It's the other way around, actually.

 

[Beeblebrox237]

Those links are what I meant by "some info", along with the "Turbo-diesel" article. I've read them yes, but I can't quite understand them.

Look, it's not hard to understand so long as you understand how a diesel engine works and how a petrol engine works.

"Because diesels do not have fuel in the cylinder before combustion is initiated, more than one bar (100 kPa) of air can be loaded in the cylinder without preignition."

Diesel is less volatile than petrol and so is less prone to knock. Knock is when the fuel-air mixture in the cylinder ignites when it isn't supposed to as a result of the heat it gains when it's compressed. See also ideal gas law: PV=RT.

Practically, this means that diesel engines can have much higher pressures in the cylinders while maintaining control over the combustion process.

"A turbocharged engine can produce significantly more power than a naturally aspirated engine of the same configuration, as having more air in the cylinders allows more fuel to be burned and thus more power to be produced."

This again is quite simple. An engine requires a certain ratio of fuel and air when they mix in the cylinder. This is related to the chemistry of combustion. Forcing more air into the cylinder without adding more fuel is generally not desirable due to the results this "lean mixture" has on the byproducts of combustion and overall efficiency, so when more air is forced into the cylinder more fuel is also added.

This means that there is more energy available to be released, and this energy is then converted into mechanical power as it drives the piston down and turns the crankshaft. This is why boost means more power.

"A supercharger is powered mechanically by the engine's crankshaft, while a turbocharger is powered by the engine exhaust, not requiring any mechanical power. Turbocharging can improve the fuel economy[57] of diesel engines by recovering waste heat from the exhaust, increasing the excess air factor, and increasing the ratio of engine output to friction losses."

This is the key part. Diesel engines are more suited to high temperature operation because they operate much more efficiently when they are warm. Turbocharging takes heat out of the exhaust and warms the incoming air. Intercooling cools the intake air to make it denser, but warm intake air can be better for a diesel because it warms the engine more quickly and keeps it at a good operating temperature.

Additionally, the extra power from turbocharging does not come at the expense of added friction, so the overall effect makes the engine more efficient because less effort is required to overcome internal losses.

 

[Eunos_Cosmo]

Diesel fuel has more energy density than gasoline/petrol. Full stop. Turbochargers just recover otherwise wasted exhaust gas heat and kinetic energy. That's all you need to know, conceptually.

 

[niky]

I've often wondered, how much boost can you usefully make out of a supercharger at the low speeds in which diesel engines typically operate?

While superchargers can boost at very low revs, turbos tend to produce more boost under load, and the power... uh... boost scales up with the engine load. It's also a shame to waste all that hot, energetic exhaust gas instead of using it to turn a turbo.

I've seen over 40 psi of boost on the pressure gauge when we hooked up a (stock) modern turbodiesel to the dyno. And that was at just under 2,000 rpm.
Damn impressive.

While I don't doubt there would be some advantages to a sueprcharger... especially since many high-powered turbodiesels still tend to have some lag... turbo still seems the way to go for diesels.

 

[LMSCorvetteGT2]

https://en.wikipedia.org/wiki/Diesel_engine#Supercharging_and_turbocharging

https://en.wikipedia.org/wiki/Diese...d_disadvantages_versus_spark-ignition_engines

Took me 60 seconds to find those two very relevant links. Is it really that hard to google something?

Not sure what it is with the high and mighty route about him asking a general question that stems from marketing that doesn't explaining. Unless your someone that actually looks at that jargon often (as an engineering student yourself would) it becomes second nature, but to someone who doesn't and needs it simplified...why not do such. It's a forum for a reason to discuss relevant topics in the appropriate area, not to be criticized cause said user asked questions and talked about relevant topics in the correct area.

 

[Leonidae@MFT]

I've often wondered, how much boost can you usefully make out of a supercharger at the low speeds in which diesel engines typically operate?

While superchargers can boost at very low revs, turbos tend to produce more boost under load, and the power... uh... boost scales up with the engine load. It's also a shame to waste all that hot, energetic exhaust gas instead of using it to turn a turbo.

I've seen over 40 psi of boost on the pressure gauge when we hooked up a (stock) modern turbodiesel to the dyno. And that was at just under 2,000 rpm.
Damn impressive.

While I don't doubt there would be some advantages to a sueprcharger... especially since many high-powered turbodiesels still tend to have some lag... turbo still seems the way to go for diesels.

Isn't superchargers boost range dependent of the pulley size? Smaller pulley> more boost earlier but dies off sooner and vice versa..?

 

[MattieM]

A turbocharger uses the remaining energy of the exhaust gasses to provide more air into the cylinder(s). More air into the cylinder means more fuel can be injected and thus more power. A turbocharger allows one to create an engine with the same power output compared to a larger naturally aspirated engine.

The remaining energy in the exhaust gasses comes from the combustion, which is created by the air and fuel (which you've payed for!) going into the engine. Without a turbocharger, this energy would be lost as it leaves the exhaust. A supercharger does the same trick, however it takes its energy directly from the crankshaft. Thats the main reason why turbochargers are used in stead of superchargers. Also, turbocharges are easier to place under the hood as it doesn't need a drive belt.

Why does all modern diesel engines have turbochargers and most petrol engines not?

This goes back to the main working principle of the diesel engine and is hard to explain in a few words. A diesel engines needs an air excess to run good over the whole rpm/load range while a petrol engine needs a constant air/fuel (AF) ratio. This constant AF ratio makes a throttle valve in the intake necessary for petrol engines. This throttle valve is a restriction (try running a few miles breathing through a snorkel) during part load conditions (<90% throttle position, full load is not used much in normal driving). A diesel engine doesn't have a throttle valve in the intake so adding a turbocharger improves efficiency over the whole range (also/especially part load conditions!).

 

[TexRex]

Isn't superchargers boost range dependent of the pulley size? Smaller pulley> more boost earlier but dies off sooner and vice versa..?

https://en.wikipedia.org/wiki/Supercharger

 

[Leonidae@MFT]

https://en.wikipedia.org/wiki/Supercharger

That didn't really answer question regarding the pulleys.

 

[TexRex]

It did the last time I checked--however, looking at it now to find specific quotes, I see that it's changed greatly in that time--apologies. I wear my ignorance on my sleeve, though, so I'll have to defer to someone better versed in the subject matter.

 

[niky]

Isn't superchargers boost range dependent of the pulley size? Smaller pulley> more boost earlier but dies off sooner and vice versa..?

Having variable pulley sizes is much more complicated (mechanically) than having variable vanes inside the turbine housing.

Also, have not seen a supercharger capable of 40 psi at under 2,000 rpm. (Would be awesome, though)

 

[Leonidae@MFT]

Having variable pulley sizes is much more complicated (mechanically) than having variable vanes inside the turbine housing.

Also, have not seen a supercharger capable of 40 psi at under 2,000 rpm. (Would be awesome, though)

Burnouts. Burnouts always.

 

[LMSCorvetteGT2]

Isn't superchargers boost range dependent of the pulley size? Smaller pulley> more boost earlier but dies off sooner and vice versa..?

All in all you were right, that's why most people who do upgrades to superchargers usually start out with the pulley because it's a simple way to get more power and at a different range.

 

[Leonidae@MFT]

All in all you were right, that's why most people who do upgrades to superchargers usually start out with the pulley because it's a simple way to get more power and at a different range.

I seem to recall that even back in the days of WWII, fighters and bombers had multi stage/gear superchargers. I guess the weight and size of that solution makes it unfeasible for cars..

 

[Adamgp]

Smaller pulley doesn't necessarily mean higher boost. To run smaller pulleys, you need other modifications to support it.

When I had my Grand Prix, stock boost was ~8psi and boost levels stayed pretty flat throughout the rev range. I dropped to a smaller pulley and boost went up to 10-11psi. Then I swapped the stock manifolds for headers and boost went back down to around ~8psi while still making more power.

 

[Leonidae@MFT]

Smaller pulley doesn't necessarily mean higher boost. To run smaller pulleys, you need other modifications to support it.

When I had my Grand Prix, stock boost was ~8psi and boost levels stayed pretty flat throughout the rev range. I dropped to a smaller pulley and boost went up to 10-11psi. Then I swapped the stock manifolds for headers and boost went back down to around ~8psi while still making more power.

So the boost increase was due to more restrictive manifolds. Pulley doesn't increase boost, it just changes the RPM range.

 

[Adamgp]

So the boost increase was due to more restrictive manifolds. Pulley doesn't increase boost, it just changes the RPM range.

A smaller pulley will increase boost if nothing else is done to the engine, since you are trying to ram more air through the engine. It doesn't necessarily alter the power band. Certain supporting mods will lower that boost back down. (exhaust/cam/higher ratio rockers/etc.)

Installing headers & free flowing exhaust (and later I put on higher ratio rockers and longer pushrods) lowered the boost back down because of much better airflow. Those mods on my Grand Prix did move the power band to higher rpms thanks to better airflow a lot less back pressure.