Author Topic: turbo vs supercharged diesel engine (Read 18456 times)

zyewdall · « **on:** September 22, 2005, 12:19:00 pm »

I was wondering why diesel engines are generally turbocharged instead of supercharged. I imagine it is more efficient to use heat in the exhaust vs HP off the crankshaft, but othan that, why?

The only supercharged diesels I have seen are two cycle designs (an old detroit diesel in a bulldozer), where the supercharger or blower is used to clear the exhaust out of the cylinder to prepare for the next power strock. And I have heard that even on these, a turbo is used for increasing power, even when it has the blower as well. Although I can't quite figure out how the two would interact...

Anyone know about this?

vwmike · « **Reply #1 on:** September 22, 2005, 12:32:29 pm »

No supercharger is going to match the efficiency of a turbo. They can be used effectively, but I don't think I would want one unless I was using it to spool a whole lot of turbo. Ask Jake about the response of the VNT and you'll wonder why anyone would bother with a supercharger.

MacGyver · « **Reply #2 on:** September 22, 2005, 01:59:44 pm »

The old Detroit Diesel 2 cycle engines require the blower to run, it's not a performance piece on them. By design they need air pumped into the cylinders.
The Roots style blowers are positive displacement devices, whatever enters them is pumped out the other side at very high volumetric efficiency.
A turbo functions the same on them as any other engine, compresses the air entering the blower, thereby allowing more air packed into the cylinders with each revolution of the blower & engine.
I'm not sure what if anything DD's used for adding more fuel when under boost. QuickTD & others here could answer that.

The downside of Roots style superchargers is the considerable amount of energy it takes to run them.

zyewdall · « **Reply #3 on:** September 22, 2005, 02:41:20 pm »

Quote from: "MacGyver"

The old Detroit Diesel 2 cycle engines require the blower to run, it's not a performance piece on them. By design they need air pumped into the cylinders.
The Roots style blowers are positive displacement devices, whatever enters them is pumped out the other side at very high volumetric efficiency.
A turbo functions the same on them as any other engine, compresses the air entering the blower, thereby allowing more air packed into the cylinders with each revolution of the blower & engine.
I'm not sure what if anything DD's used for adding more fuel when under boost. QuickTD & others here could answer that.

The downside of Roots style superchargers is the considerable amount of energy it takes to run them.

So on the Detroit Diesel, the blower is not a supercharger per se, but is more properly seen as a volumetric device to pump air in and out of the cylinders, which can't do it themselves in the two stroke design. I did notice that it's designed with a positive displacement design like a giant gear pump.

But why does the new Nissan Xterra (gasoline) have a supercharger instead of a turbocharger? It's a four cycle engine. Is it cheaper than a turbo, even though it uses more power, or does it affect the torque/power curve of a gas engine differently than it would a diesel?

vwmike · « **Reply #4 on:** September 22, 2005, 02:52:40 pm »

The factory is after 2 things in the design - Overall power increase, and a flat power curve. It may also be that they simply couldn't package a turbosystem under the hood. Most of the time it's just easier to throw a supercharger on top. But, their primary concern is making a very driveable car engine with good torque off the line and a steady increase throughout the rev range. Ultimate power isn't nearly as much of a concern as driveability. If all car manufacturers were out there to get as much power out of any given configuration then we'd all be twiddling our thumbs with nothing to do.

MacGyver · « **Reply #5 on:** September 22, 2005, 08:25:37 pm »

Quote from: "zyewdall"

But why does the new Nissan Xterra (gasoline) have a supercharger instead of a turbocharger? It's a four cycle engine. Is it cheaper than a turbo, even though it uses more power, or does it affect the torque/power curve of a gas engine differently than it would a diesel?

Cheaper? I'm not sure. Anymore with 'complex' VNT turbos, and 'simple' Paxton kits for Ford products & such, the cost line is kinda blurry.

Traditionally superchargers on gas cars have been seen as more of a hardcore performance part than a turbo, ie 6 second 1/4 mile cars run blowers not turbos, so Joe Public's gotta have one on his street car to be the best :wink:
Boost can be had right off idle and instantaneously with a blower. Turbos are slower to respond (traditionally) and vwmike is right, bolting a blower on an existing system is seen as quick power.

Personally I think of turbos as more reliable for the long term, but that may be out of date given advancing technology...

QuickTD · « **Reply #6 on:** September 22, 2005, 08:49:27 pm »

Quote

6 second 1/4 mile cars run blowers not turbos, so Joe Public's gotta have one on his street car to be the best

Turbo's are not used on top fuel/alchohol 1/4 mile cars due to the severe weight penalties placed on turbocharged cars. Back in the 70's there were some turbocharged top fuel cars. Turbocharger technology was expensive at the time and the turbo cars were totally dominant, so the NHRA imposed weight penalties on the turbo cars to "keep the sport competative". With the weight penalties the turbo cars were no longer competative and they quickly faded away. The weight penaties for turbo's are still in the rule books.

I don't understand superchargers on street cars. Throttle response is a bit better than a turbo, other than that they are the clear loser in every other area of performance/efficiency. "Supercharged" sounds good in the brochure though, something like "hemi"...

vwmike · « **Reply #7 on:** September 22, 2005, 09:43:51 pm »

How many people do you think actually even know what Hemi means? Or for that matter, how many people actually know that their vacuum cleaner (or any other product which uses the name) isn't turbo?

jackbombay · « **Reply #8 on:** September 22, 2005, 10:47:24 pm »

Quote from: "vwmike"

How many people do you think actually even know what Hemi means?

I started a poll about that on TDI club a while back actually, I bet few Americans do.

Quote from: "vwmike"

Or for that matter, how many people actually know that their vacuum cleaner (or any other product which uses the name) isn't turbo?

My computer has a turbo on it. :roll:

[/off topic]

Good reading!

vwmike · « **Reply #9 on:** September 22, 2005, 10:50:36 pm »

Well, in that case, I have 2 turbocharged shelves, a turbocharged cardboard box, and a turbocharged concrete floor.

Master ACiD · « **Reply #10 on:** September 22, 2005, 11:00:53 pm »

i think hemi refers to the hemispherical combustion chamber shape. like a cheerios bowl.
my 2 stroke dirtbike has a hemi head.
my 1.6 vw has not a hemi head. it seems to be more or less flat.

i know that alot of people think hemi means to have the sparkplug centrally located. just because it was quite odd back in the day for an american engine to have a sparkplug going through the middle of the head, sence most at that time has the plug off to the side, but if that was the definition, any 4 valve pentroof engine would be considered a hemi. but any 4 valve pentroof isnt a hemi, its a pentroof.

QuickTD · « **Reply #11 on:** September 23, 2005, 04:35:12 am »

Quote

but any 4 valve pentroof isnt a hemi, its a pentroof.

And a pentroof is a superior design. Wide, flat combustion chambers burn quickly and cleanly and produce the best power. The hemi with it's bowl shape, steeply angled valves and tight squish areas around the perimeter leads to valve shrouding, valve interference and detonation problems in high performance engines. It represented the state of the art in the 50's, (when everything else was a flat head or a wedge) but we can do better now. Its a buzzword to trick those baby boomer types into buying a dodge.

Speaking of odd uses for terms that no one knows the meaning of, I recently saw an ad for a "hemi" power drill. From what I could tell, it was not powered by a gasoline engine, go figure...

zyewdall · « **Reply #12 on:** September 23, 2005, 09:36:00 am »

Gotta love the marketing people..... I'm an engineer, and it's always fun trying to explain the engineering behind something that the marketing people described, without just saying "its completely made up".

BTW, I love the Hemi picture...

BlackTieTD · « **Reply #13 on:** September 26, 2005, 08:10:19 am »

Quote from: "QuickTD"

Throttle response is a bit better than a turbo, other than that they are the clear loser in every other area of performance/efficiency.

throttle response is the only reason to run a supercharger over a turbo. IMO i would much rather run a supercharger over a turbocharger. i haven't had the pleasure of driving a quick-spooling setup like jake's VNT... but i have driven numerous turbo-charger cars. more 1.8Ts than i can count, 2.7L twin turbo audis, XR4Ti Merkur, tons of TDIs and IDI turbo cars... and none of them come close to matching the sheer pull at the dip of the throttle of my old G60 setup. original g-lader design rebuilt and ported.. it's not even an efficient charger and personally i'd take that over any of the turbo setups i've driven. it might not have as much peak HP and the efficiency is no contest... but go drive a supercharged car, hammer it off the line or out of a corner and try to hide your smile :wink:

toomanycars · « **Reply #14 on:** September 27, 2005, 02:01:00 pm »

No personal experience here, but here in Oz Holden (GM) produced a supercharged Commodore V6 throughout the 90s. The performance figures exceed those of the V8. Mazda have had supercharged diesels in their 626 (Capella) cars. My memory is a bit vague here (must have been late at night!) but I think I read something about VW producing a turbo+supercharged Golf. The supercharger gets the thing going to defeat turbo lag, and then the turbocharger takes over. As I say, my memory is sketchy.