What's Up with all these non MB Diesel Threads?

The turbocharged models are NOT supercharged AND turbocharged. The roots type blower used on the 2 cycle detroit does NOT serve as a supercharger. It is used as a blower. The difference is that the blower is simply serving to purge the exhaust and introduce fresh air. This is still basically a normally aspirated engine.

Once the turbocharger is put on top of that, it is a turbocharged engine.

When a Detroit Roots type blower is adapted to the top of a gas engine, it is THEN acting as a supercharger in that it is no longer a normally aspirated engine.

[babymog]

The turbo models ARE turbocharged and supercharged, very common. AFAIK they never produced a 71 or 92 series DD without the blower. The marine engines are typically a turbo for each 3-4 cylinders and intercooled (between the turbos and the blower).

Runaways are a problem with the 2-stroke detroits. If the blower seals go bad, the engine can run on the lube oil. There is a shutoff flap in most (emergency-stop) to shut them down in a runaway, problem is if the blower seals are bad enough, it can suck enough oil and air past the seals to stay running.

[lietuviai]

Originally Posted by babymog

Runaways are a problem with the 2-stroke detroits. If the blower seals go bad, the engine can run on the lube oil. There is a shutoff flap in most (emergency-stop) to shut them down in a runaway, problem is if the blower seals are bad enough, it can suck enough oil and air past the seals to stay running.

I believe that was the case of the DD that I was told about. It would not shut down and the rpms suddenly climbed to where the engine self destructed.

[babymog]

In further discussion about the blower/supercharger, it comes down to the definition I guess. Since the output of the blower is always above atmospheric, it is compressing air, and seems to me like it would technically be a supercharger.

Granted its primary function is to provide enough pressure to blow the exhaust out of the cylinders, but it has to compress air to do so.

[Brian Carlton]

Originally Posted by babymog

In further discussion about the blower/supercharger, it comes down to the definition I guess. Since the output of the blower is always above atmospheric, it is compressing air, and seems to me like it would technically be a supercharger.

Granted its primary function is to provide enough pressure to blow the exhaust out of the cylinders, but it has to compress air to do so.

From my understanding of the engine, the blower is required at low speed to enable the engine to start and run decently. At high speed, the turbo provides more air than the supercharger can possible handle and it's basically freewheeling. It's been suggested that the supercharger could be provided with an overrunning clutch so that no horsepower is utilized above a specific rpm, but DD never introduced that. I'd be curious as to the power required to spin it at high speed when it's doing minimal work.

[engatwork]

I knew the answer on the DD but have just now been able to log on. A few of the chip and bark pushers at work run the v8's.

Originally Posted by babymog

The turbo models ARE turbocharged and supercharged, very common. AFAIK they never produced a 71 or 92 series DD without the blower. The marine engines are typically a turbo for each 3-4 cylinders and intercooled (between the turbos and the blower).

Runaways are a problem with the 2-stroke detroits. If the blower seals go bad, the engine can run on the lube oil. There is a shutoff flap in most (emergency-stop) to shut them down in a runaway, problem is if the blower seals are bad enough, it can suck enough oil and air past the seals to stay running.

Mog,

I guess this is best explained by trying to put forth some definitions.

The Roots Blower is a positive displacement PUMP.

Usage of this pump on the DD is as a BLOWER. With the PUMP being of positive displacement, it's size and effect can be precisely calculated.

Usage of this pump on a gas engine to boost pressure beyond the Normal Aspiration level uses the same pump as a SUPERCHARGER.


The blower on the DD provides fresh air through the valves, through the cylinder and exiting the exhaust ports. The blower basically serves as the intake and exhaust stroke all in one. It's purpose in this application is not to raise pressure beyond normal aspiration, although there may be a slight amount of that under certain situations. There should not be because the exhaust ports and the cam actuated valves close at virtually the same time.

Confusion is inserted because the same PUMP is used on gas engines to raise the pressure above normal aspiration, thus serving as a supercharger.

SOoo.... the "Supercharger" that you refer to on the Turbocharged DD, is not a "Supercharger," rather it is a blower serving to purge the spent charge and introduce fresh air at the same time. The turbocharger on top of the "blower" raises the pressure above atmospheric at the point of entrance into the blower.

It would be possible to use the pump as a supercharge on the DD by oversizing this positive displacement pump. The problem with that approach is that it takes energy from the crankshaft to turn the pump, thus making this a less efficient way to boost air volume as opposed to a turbocharger. The turbocharger is driven by wasted energy, thus being somewhat free energy.

Effectively this makes for a turbo charged engine with the pump only acting as the intake/exhaust rather than an air volume booster. To LOOK at this engine you would see the BLOWER and the turbocharger. Unless a person understands the USAGE of the pump and the turbocharger, it would be easily confused as having a supercharger AND a turbocharger.

Adding to this confusion is the fact that many of us grew up seeing a DD blower on top of a gas V8 where it was used as a supercharger, but often mistakenly called a blower.

The terms BLOWER and SUPERCHARGER refer to the USAGE of the PUMP.

[babymog]

I understand all that you say Larry, my only point is that regardless of its primary purpose (to create proper air turnover in the cylinders), it is a compressor as well as a pump (since the ouput pressure is greater than the input pressure, above exhaust pressure and above ambient), it can technically be also referred to as a supercharger, albiet only as a secondary result of its primary purpose. I realize that DD doesn't intend it as such, just the only way it can do its job.

And I've never had a DD blower on a gas V8, found out that the 8V71 blowers are popular for that though. All my experience is on DD engines, I've had 8V92TAs and have wrenched on 8V71ITIs and S-60s for friends.

[t walgamuth]

I believe Larry's point is that the blower does not increase the cylinder pressure but is only used to scavange out the exhaust gasses.

If it does not increase the cylinder pressure it is not a supercharger.

(Right?)

[Brian Carlton]

Originally Posted by t walgamuth

I believe Larry's point is that the blower does not increase the cylinder pressure but is only used to scavange out the exhaust gasses.

If it does not increase the cylinder pressure it is not a supercharger.

(Right?)

If you want to evacuate a cylinder, you can scavenge the air out with a vacuum type pump........or you can force it out with a pressure type pump........aka blower.......aka supercharger.

In the case of the blower, the cylinder gases won't move out unless the pressure in the cylinder is greater than ambient pressure.

A Roots blower is a positive displacement, one to one air mover. If there is no resistance on the output side, it will build no pressure, it will only move air.

Here's how the DD works:

The cylinder head has valves much like any cylinder head. The bottom of the cylinder has ports that are open when the piston is near the bottom of the stroke.

Once the piston gets near the bottom, exposing the exhaust ports, the valves open. At this point the air is free to travel through the cylinder and out the exhaust ports, forcing the spent charge out and fresh air in(a scavenging process.) Remember a diesel, ANY diesel does not breathe in an air/fuel mixture, it ONLY breathes in fresh air.

With the fresh air in the cylinder, the piston moves up sealing the exhaust ports as the valves close, building high pressure until near TDC, fuel is injected into the hot air causing ignition which causes combustion which pushes the piston down.

SO, with the valves open and the exhaust ports exposed, the pressure in the intake manifold created by the blower is free to flow through the cylinder.

The blower is positive displacement, thus it puts a precise volume of air through with every turn. This volume of air processed by the blower is matched to the volume of air required to scavenge the cylinders.

The scavenging process supplies enough fresh air into the cylinders such that it is equivalent to the amount of fresh air in a four cycle diesel engine.

[babymog]

... and if that intake air is not at a pressure greater than the exhaust-side pressure, no air will move.

Of course the intake pressure is greater than the cylinder and exhaust pressure. The valves were closed allowing intake manifold pressure to build. The issue is NOT about pressure. The issue is about volumetric efficiency. If the pressure that scavenges the cylinders manages to put more volume in the cylinder than the displacement of that cylinder, then it becomes above normal aspiration level. The volume available from the positive displacement blower is calculated to provide something in the area of 100% volumetric efficiency.

The turbo charger then applies increased pressure at the entrance to the blower that increases the VE significantly above 100%. That is the difference.

One of the differences that might not be obvious is that a normal 4 cycle engine, gas OR diesel SUCK their charge into the cylinder, while the 2 cycle DD BLOWS in the same amount of air. The fact that the air is under pressure rather than vacuum in order to fill the cylinder does not make it a supercharged engine. If you put a bigger blower on such that it significantly added more than cylinder volume of air, THEN you could call it supercharged.

[Brian Carlton]

It appears that the terms "blower" and "supercharger" can be used independently, without the end result being the determining factor:

http://en.wikipedia.org/wiki/Roots_type_supercharger

It's clear that the DD design doesn't raise the cylinder pressure by any significant amount above atmospheric. It's not clear that the term "supercharger" cannot be used for such an application.

As used on the DD, it is NOT used as a Supercharger. If it were heavily oversized it could be, but that would require excessive torque taken from the crankshaft.

What one must understand about a 2 cycle engine is that the incoming charge can not be SUCKED into the cylinder. It must be BLOWN into the cylinder. The positive displacement Roots blower as fitted to the DD is calculated to blow in the equivalent air volume as would be SUCKED into a four cycle equivalent.

The term Supercharge implies the introduction of ADDITIONAL air volume beyond the cylinder displacment.

The same blower is commonly fitted to a gas engine and driven at a rate that will supply more than 100% volumetric efficiency and it serves as a Supercharger.

The same goes with the DD. If you were to change the drive gear ratio or use a larger displacement Roots blower such that it would introduce more than 100% of the cylinder volume in fresh air, and you could THEN term it as a supercharger.

In it's standard form as used on a DD, it is NOT designed to exceed 100% volumetric efficiency, THUS it is not used as a supercharger. It is simply serving as the intake/exhaust stroke, thus serving as a blower.

In a 2 cycle gas engine such as a chain saw or dirt bike, the charge is forced into the cylinder through the transfer ports. These ports connect the crankcase area to the open cylinder ports so that the downward movement of the piston forces the air/fuel charge through the transfer ports to fill the cylinder. In this case the charge is being blown or forced into the cylinder.

The 2 cycle engine whether a chain saw or a DD, requires a means of forcing the charge into the cylinder since it makes no vacuum.