Why are diesels turbocharged . . .

My son asked today why the city buses have turbos and not superchargers. I have no idea -- and I can't recall ever reading about a supercharged diesel (although someone surely makes one). So why is it that most diesels are turbocharged?

Rob

Turbos are better at delivering boost when actually needed ( based and exhaust gas heat and output - hence load ), rather than based on RPM.

Two possible reasons off the top of my head, completely free from any actual engineering knowledge or anything:

1. Yer typical classic diesel is BIG, meaning lots of exhaust, meaning lots of exhaust available to be used to turn the compressor.

2. Yer typical classic diesel works hard for its living, so using the "free" power from the exhaust is a more attractive economic proposition than adding an additional task to the crankshaft's job list.

There might also be something relevant in the relatively low rpm of the typical diesel, but that would require me to have actual engineering knowledge, which I don't.

But somebody on here will!

I would say that the lower engine speed would have more to do with it. Yeah you can change pulleys and make more boost, but turbos are free boost, and can make more with less engine speed.

Diesels are also remarkably efficient at idle, making little heat and using very little fuel. The low, almost no boost at idle nature of a turbo, really helps a diesel maintain its efficientcy, which is the biggest reason why industry switched over to them.

have you driven a non turbo diesel?

My thought process is why not - Diesel engines are inherently built for boost.

Diesels are turbocharged because otherwise they make all torque and no horsepower!

Wait, what was the question?

David

Didn't a lot of older diesels have superchargers? I swear I remember the old 6-71 type blowers were originaly from heavy truck applications.

Detroit Diesel used to supercharge their old two stroke diesels. The term 8-71 or 6-71 is in reference to the blowers that they put on them. I believe, from what I can remember, to stramline production, all the engines had a single cylinder size of 71 CI, and if a supercharger was an 8-71, it could feed eight cylinders. A V12 detriot would have two 6-71 superchargers on it.

Someone somewhere figured out that these would work well on hot rods, and the rest is history.

I am pretty sure the 6-71 were used on the exhaust.

Diesels are perfect for boosting since they can't ping (they are constantly pinging). Turbos are FAR more efficient than Superchargers which use a large amount of HP to turn, turbos take very little from the engine to turn (the exhaust energy is almost free). Lyschom screw type SC are more efficient but still not close to a turbo.

it's one less belt to have to worry about later.

altitude compensation..

Superchargers are poor at it.

The other answers are not bad, but the altitude compensation is the key... I can 'splain more if needed.

Yes, for all those flying diesels It's true that superchargers are actually affected more than naturally aspirated cars by altitude changes and turbos are not (up to a point), but other than big trucks hauling from Nebraska to Colorado that's not a huge factor.

The low engine speed really isn't important - that just means you would have to select the correct supercharger for the application. I think it's actually a really good questions - the efficiency factor is quite likely a large part of it, and a slightly soggy response time isn't really an issue with a diesel as they're often run in a narrow rpm range.

Diesels also tend to run at steady speeds too.

Two strokes also need all the help they can get in scavanging the old exhaust out

Keith wrote: Yes, for all those flying diesels It's true that superchargers are actually affected more than naturally aspirated cars by altitude changes and turbos are not (up to a point), but other than big trucks hauling from Nebraska to Colorado that's not a huge factor.

Altitude compensation was the original reason the two were mated successfully way back in the 50's(both superchargers and turbos to diesels). Now.. Since then we discovered that turbos have significant benefits vs supercharging.

1. being the use of "waste" heat and energy therefore increase the efficiency of the system. Huge savings from turbo to supercharger.
2. The decrease in intake restrictions.. It can be argued that a twin screw supercharger is more of an impediment to intake flow than an off boost supercharger.
3. The ability to then cool the charge before entering the cylinders and further down the line the ability to install EGR systems easier.. (It can be done with superchargers but the piping gets silly.)
4. The simplicity of the machine. Current Holset turbos have a 1M mile warranty when paired with a cummins engine. Find me a supercharger that will do 1M mile warranty.(I guess since I used to work in warranty you could say.. Find me a turbo that will do 1M miles.. Ha Ha.)
5. To go along with point 1.. Superchargers require a relatively "high" amount of energy to run. while a larger turbo may require 5-10hp to run(it is a restriction).. A large supercharger can take 30-50hp..

Altitude compensation was the original reason and is still the reason that turbos are paired to diesels. The engines run best lean and at anything above death valley you can get massive changes in mixture due to the air density..

If you think a regualr diesel has a very small usable rev range, try driving an old 2 stroke detroit.. There is a reason why they had 653 forward gears...

http://www.dieselpowermag.com/features/0802dp_detroit_diesel_dd15_diesel_engine/index.html <-- Thats the future though... The technology still isn't perfect but... 50 hp and 100 lb-ft back to the crank. I worked on that program and can talk to it if you want.

Non OTR diesels, remeber boats and trains started it all, used turbos just to simply make more power. Superchargers weren't reliable enough in the 1930's and didn't move enough air.

Thanks for the insight, that's really interesting. It's funny that there's such a clear "correct" choice for diesels but not for gasoline engines. I'm thinking of passenger vehicles here, of course.

The answer, to add more win.

And yes Detroit Diesel did to turbo diesels, supercharged diesels and turbo AND supercharged diesels.

Keith wrote: Thanks for the insight, that's really interesting. It's funny that there's such a clear "correct" choice for diesels but not for gasoline engines. I'm thinking of passenger vehicles here, of course.

2 things hurt a spark ignited gasoline turbo'd engine..

1. a throttle plate (big restriction)
2. They operate at higher temp ranges, higher exhaust temp ranges and greater transient speeds.

Would like to hear more about the program (thread of it's own?) Some interesting tidbits besides the compound turbochaging: hollow aluminum camshafts compacted carbon - iron head variable pressure injectors (I assume piezo units?)

Carter

erohslc wrote: Would like to hear more about the program (thread of it's own?) Some interesting tidbits besides the compound turbochaging: hollow aluminum camshafts compacted carbon - iron head variable pressure injectors (I assume piezo units?) Carter

I only really know about the compound turbo bit..

A better question is, why isn't everything else turbocharged?

My winter project.

Trans_Maro wrote: A better question is, why isn't everything else turbocharged? My winter project.

oodles of win

Diesels are turbocharged because otherwise they make all torque and no horsepower!

Heh, my experience has been that diesels are turbo'd because otherwise they make NO torque AND NO horsepower.

We had a non-turbo diesel Chevy for a work truck at one time.

My boss hated it when I told him that it drove just like my gas powered truck with the parking brake on.

Shawn

i think a good simple way to put it is that superchargers deliver lots and lots of torque and turbos are better for horsepower. diesels aready have enough torque, what they need is more horsepower. also they move so much air that an air driven (exhaust driven, whatever) forced induction system is truly free power

aircooled wrote: I am pretty sure the 6-71 were used on the exhaust.

A 6-71 is a supercharger. I'm not sure where you're going with this.