Thread: Supercharge a 4BD1?

i dont understand why more diesels arent supercharged, sure theyre less efficient than a turbo but if the engine is to be used over a short rev range (like most deisels) then you could overdrive them quite a bit to get boost up and get huge torque figures off idle, for a 4l engine an eaton m90 should match pretty well. maybe superchargers just arent as reliable as their turbo cousins.
im talking about positive displacement superchargers here centrifugal is a whole different game

Originally Posted by mopar

i dont understand why more diesels arent supercharged, sure theyre less efficient than a turbo but if the engine is to be used over a short rev range (like most deisels) then you could overdrive them quite a bit to get boost up and get huge torque figures off idle, for a 4l engine an eaton m90 should match pretty well. maybe superchargers just arent as reliable as their turbo cousins.
im talking about positive displacement superchargers here centrifugal is a whole different game

Superchargers are less reliable and much more expensive - and need to be where they can be driven, whereas turbos can be put where there is room, more or less without restriction. If superchargers were worthwhile as you suggest, I suspect you would find at least a few on the market.

John

Many diesels in the past were supercharged and worked quite well. Cost is probably the main reason why the practice is virtually defunct.

As to blowers and superchargers, Commer Knockers ran with a significant degree of supercharge as well as being exhaust scavenged by the Roots blower. One knew when a blower overhaul was imminent by a loss of performance, increased fuel consumption, and more fuel smoke.

Detroit two strokes ran with barely any boost from the Roots blower, this being purely a supplier of intake air and exhaust scavenging. Turbochargers were fitted for boost in these engines. Later DD's and EMD's had blower bypass systems that kicked in to bypass the Roots blower once the turbocharger was passing sufficient wind to provide both boost and scavenge. Some EMD's had an gear driven centrifugal blower that disengaged the gear train and became a turbocharger when speed was suffiicient

One that comes to mind as a very neat installation was the supercharged Buda in Allis Chalmers 45 motor graders.

People tend to forget that a turbocharger has to provide sufficient added power to overcome the significant exhaust restriction it causes.

Originally Posted by mopar

i dont understand why more diesels arent supercharged, sure theyre less efficient than a turbo but if the engine is to be used over a short rev range (like most deisels) then you could overdrive them quite a bit to get boost up and get huge torque figures off idle, for a 4l engine an eaton m90 should match pretty well. maybe superchargers just arent as reliable as their turbo cousins.
im talking about positive displacement superchargers here centrifugal is a whole different game

...and a 4bd1 will rattle them to bits, and without intercooling just heat up the inlet air, at lower boost pressures than a turbocharger with an intercooler. Set up right, a turbocharger installation on a diesel engine is about as efficient as it gets IMO.

JC

Originally Posted by mopar

i dont understand why more diesels arent supercharged, sure theyre less efficient than a turbo but if the engine is to be used over a short rev range (like most deisels) then you could overdrive them quite a bit to get boost up and get huge torque figures off idle, for a 4l engine an eaton m90 should match pretty well. maybe superchargers just arent as reliable as their turbo cousins.
im talking about positive displacement superchargers here centrifugal is a whole different game

Mainly because they supply boost in a way that's completely unrelated to what the engine needs.
A turbo that's well matched to the engine provides more boost when needed and none when it's not. Which is pretty much ideal. A positive displacement supercharger always provides the same amount of boost, so massive parasitic loss and will never warm up at lower loads and a real hit on fuel economy.

The "huge torque off idle" isn't going to happen and it's not to do with boost.
4 cyl engines get extremely rough when asked to deliver a lot of torque at low revs (my T25 turbo could produce 20psi by 1400rpm). If you want to produce a lot of torque below 1500rpm then you need to triple the inertia of your flywheel and get more cylinders firing.

For parasitic power, to produce 15psi boost at 2000rpm requires over 20kw of crank power to turn the supercharger. A turbocharger can do the same thing while only needing a few psi more drive pressure than the boost it provides.
In the best operating points a turbo can provide more boost than backpressure.

Lots of people have tried to supercharge 4 stroke diesels, none have stuck with it.

The point about efficiency of a turbocharger is that it puts to use much of the back pressure that is necessarily present from the exhaust system, and further, by expanding and hence cooling the exhaust, makes use of what would otherwise be wasted heat energy. (One of the points I have always wondered about is why nobody has ever run auxiliaries such as alternators off an exhaust turbine - probably because it is a lot more expensive than a belt, although it would increase efficiency. An alternator could be on the same shaft as the turbocharger.)

John

Originally Posted by justinc

...and a 4bd1 will rattle them to bits, and without intercooling just heat up the inlet air, at lower boost pressures than a turbocharger with an intercooler. Set up right, a turbocharger installation on a diesel engine is about as efficient as it gets IMO.

JC

im not saying that a supercharger is more effiecient, not by a long way, just that a supercharger delivers it in my preferred range with no lag. although turbos on diesels spool up alot quicker than petrol turbos anyway.
its like comparing a 750cc bike to a 2l car, the bike engine makes more power but the car engine has it where its needed

Originally Posted by Dougal

Mainly because they supply boost in a way that's completely unrelated to what the engine needs.
A turbo that's well matched to the engine provides more boost when needed and none when it's not. Which is pretty much ideal. A positive displacement supercharger always provides the same amount of boost, so massive parasitic loss and will never warm up at lower loads and a real hit on fuel economy.

The "huge torque off idle" isn't going to happen and it's not to do with boost.
4 cyl engines get extremely rough when asked to deliver a lot of torque at low revs (my T25 turbo could produce 20psi by 1400rpm). If you want to produce a lot of torque below 1500rpm then you need to triple the inertia of your flywheel and get more cylinders firing.
how does that apply to 4cyl only? theres a lot of low revving 4cyl engines.
doesnt it depend more on your bore/stroke ratio

For parasitic power, to produce 15psi boost at 2000rpm requires over 20kw of crank power to turn the supercharger.
where did that figure come from? is it a guess or an actual figure calculated from the flow figures required for a 4bd1?

A turbocharger can do the same thing while only needing a few psi more drive pressure than the boost it provides.
In the best operating points a turbo can provide more boost than backpressure.
Im assuming thats something to do with the air being slightly cooler on the other side of the turbo? and increasing the speed?

Lots of people have tried to supercharge 4 stroke diesels, none have stuck with it.
the only application on diesels i have seen it widely used is pull tractors, but the supertrucks run turbos.

At the end of the day, yes turbos are a more efficient less wastefull power adder but superchargers do have their benefits too.