AK RAM

This is one area that I have not been able to get a definitive answer on. Manufacturers/retailers dance around the subject when I ask.

When talking turbo drive pressures, which is generally better for healthy turbo life, high drive pressures or low drive pressures? Can anyone help me out here?

RustyJC

In general terms, low drive pressures are preferable to high drive pressures (and associated higher EGT's). If the turbocharger turbine and compressor are more efficient, less drive pressure (actually, exhaust gas horsepower, or EGHP) is required to produce a given amount of airflow at a given compressor head (compressor discharge pressure minus compressor suction or inlet pressure). The lower the drive pressure for a given boost, the more efficient scavenging will be during valve overlap as well - ideally, drive pressure should be less than boost pressure, but that won't always be the case if the turbocharger is operating far off the best efficiency island on its map.

Rusty

qzilla

Well you don't want low drive pressure and the higher the drive pressure the less efficient everything works. What you shoot for is around a 1:1 ratio. This will never happen. If you get around 1:1.15 or so then you are doing good with a modded truck.


Drive pressures are tough to monitor as the pulsing of the engine is hard to stabilize. It is best to use a wide sweep gauge, say 0-500 if you can get it, that has liquid in it. This will help stabilize the pulsing so you can read it.

RustyJC

With all respect, you want the lowest possible drive pressure for the target airflow/boost you're seeking. Another way of saying this is that I want to expend as little EGHP as possible to produce the airflow I need since more EGHP costs me BHP on the engine side through pumping losses on the exhaust stroke due to the higher "back-pressure" each cylinder sees. Higher drive pressures also mean higher EGTs, all else being equal.

High efficiency turbochargers that we manufacture for our large industrial 2-cycle engines always have drive pressures lower than boost pressures across their entire operating range; otherwise, the port-scavenged pure turbocharged engines wouldn't run. The only thing that scavenges the cylinders on these 2-cycle engines is the pressure differential between the inlet ports and exhaust ports, which (after pressure drops) relates back to compressor discharge pressure (boost) minus turbine drive pressure. These turbochargers produce airflows of 220% or more of theoretical engine displacement on emission-controlled 2-cycle engines.

If drive pressures are consistently higher than boost pressures, it just means that (1.) the turbocharger isn't very efficient by design or (2.) it's being applied well off of its design map. Situation #2 would include BOMBing applications where the engine becomes more and more of a restriction to airflow as we try to push more air through fixed geometry ports & valves - the result is too much head (insofar as the turbo is concerned) for the flow being produced. And, yes, at 34 PSIG maximum boost, I'm sure my poor old HX35W-12 is in this situation.

Rusty

crabman

Heh, try and find a bombed truck with a drive pressure lower than 1:1. Your in for a long hunt. For our purposes the closer you get the better off you are but you aint getting there with any of the current stuff.

RustyJC

I believe that's what I said, right? See below:My point is that anything that we can do to lower drive pressure for a given airflow is a good thing - which ties back to the original poster's question.

Rusty

qzilla

Remember that a 2 stroke and 4 stroke are totally different animals. For that matter gas and diesel are totally different.

RustyJC

Ummm....yes, our company has a long history (founded in 1833) as a manufacturer of 2 cycle and 4 cycle spark gas engines as well as 4 cycle dual fuel and diesel engines, gas turbines, reciprocating and centrifugal compressors, turbochargers and control systems. Having spent over 30 years here, 25 of those in technical management, I'm vaguely aware of the differences between engines and how turbochargers are designed and sized for each.

Rusty

Push Rod

I'll chime in, and go with Rusty on this one. When selecting turbochargers, or designing a turbo system, you want to strive for the lowest possible drive pressure you can get. As Rusty said, less drive pressure = better scavenging.

The reality of the situation on our trucks is that no-one has come up with a single charger that gets us close to 1:1 and still maintains any kind of decent spool-up. We're simply asking far too much out of most of these chargers. ie: we're running them out of their most efficient area of operation. Most larger chargers, such as HX40s and Schwitzer chargers are designed to be run at around 20 - 25PSI of boost.

The 3rd gen trucks are especially bad for this. In order to make drivable power on a 3rd gen, you must maintain decent spool-up. The ECM relies on delta boost to determine a large part of its fueling parameters. No change in boost = little additional fuel. So you stuff a small turbine on a large compressor, your drive pressures get right out of control due to alot of mass flow. We've seen drive pressures of 100 PSI+ at 50 PSI of turbocharger boost on these trucks. With an imbalance such as this, EGTs skyrocket and the truck will fall on its face at higher RPMs. Its also extremely hard on the turbo. High drive pressure + high EGTs = dead turbo.

The best solution we've seen on the 03s is a reasonably tight turbine side, and a large external waste to eliminate some of the flow and relieve drive pressures. But we still haven't seen 1:1 on a single charger truck yet.

Rod

crabman

Never said he was wrong. Just that it aint happening with the current gear. My turbo runs at 40 psi of boost on a 60 psi drive pressure and its about the same with the DD turbo. I havent seen much better results reported by most folks on one of these trucks and thats a long way from 1:1.

RustyJC

Which is the same thing Rod and I are saying. Our "current gear" isn't really designed for our applications on a BOMBed truck - my lightly modified little 347 BHP truck included! I'm pulling 34 PSIG boost from an HX35W-12 that was sized for 21 PSIG boost and airflow rates to support 245 flywheel BHP, so I'm way off the design efficiency map.

I think the message is that, insofar as theoretical improvements are concerned, there's some real opportunity lying out there. I was noticing in a recent issue of either Road & Track or Car and Driver that one of the automotive manufacturers was about to come out with a parallel twin turbo setup that utilizes a small turbo to provide quick spool-up at low speeds/loads. When the drive pressure gets too high on the small turbo, a valve in the exhaust starts to open to admit drive energy to a large turbo which takes over for high speeds/loads. Interesting concept for "single-turbo" trucks - pick the right turbo that's sized for your requirement.

Rusty

Push Rod

Its really unfortunate that there isn't a larger market or a little more money available in the current market that would spark some compressor development for our application. I'm not aware of a single charger available for our trucks that doesn't start out its life as a medium- or heavy-duty truck charger and get modified from there. You're already starting behind the eight ball, as you've got a compressor wheel that is designed for 20-25PSI of boost, and no more.

Rod

crabman

Ok, I think we all agree that we agree. My point was that the 1;1 or less thing isnt applicable to us right now. As close as you can get is the best you can do for the time being. BTW Rod have you checked your PMs lately?

Push Rod

Hmmmm, yep. I haven't seen anything from you though. Did I miss one?

Rod

crabman

Yep, i sent it better than a week ago.