pz@emacs.UUCP (Paul Czarnecki) (01/02/86)
<fnord>
I have a question about the functioning of turbo and super chargers.
First, be warned! The early part of this article contains factual
errors, do not skim it and then flame me saying "You're wrong!", I know
I am, that is why I want the answers.
Both turbos and supers compress air and put it inside of the engine.
The reason why you do this is that you need more oxygen to burn in
addition to more fuel. A super charger is mechanically coupled to the
engine. If the engine is turning a x rpm then the super charger is
turning at sc(x) where sc() is linear. The same is not true of
turbos. The turbo is fluidly coupled (air is a fluid) to the engine.
I had thought that `if the engine is turning at x rpm then the turbo is
turning at tc(x) where tc() is a linear function ONLY AT TIME t sub zero
after the engine reaches x rpm.'
This is the source of turbo lag. The turbo must `catch up' to the
engine.
Well my boss let me drive his car (Saab 900 Turbo w/16 valves and APC,
btw, this is a nice car (too bad it's been broken into twice in the past
year)) the other day and it became obvious that the above is not true.
Take the case where you are charging down the road at 4,000 rpm and you
let up on the throttle without depressing the clutch. Under the above
theory the turbo would continue spinning at tc(4000) since the engine is
still at 4,000 rpm. However the boost guage indicated an IMMEDIATE drop
in boost.
QUESTION:
why is this so?
Any takers?
(super charger trivia -- back in the 50's a lot of MG owners put
superchargers on their cars. Name the city that made (most) of these
and name at least one other metal sporting good part that this city
made. hint: it's near my hometown :-)
pZ
--
-- My wife ran off with my best friend, and I miss him.
Paul Czarnecki
Uniworks, Inc. decvax!{cca,wanginst!infinet}!emacs!pz
20 William Street emacs!pz@cca-unix.ARPA
Wellesley, MA 02181 (617) 235-2600ark@alice.UucP (Andrew Koenig) (01/03/86)
> Take the case where you are charging down the road at 4,000 rpm and you > let up on the throttle without depressing the clutch. Under the above > theory the turbo would continue spinning at tc(4000) since the engine is > still at 4,000 rpm. However the boost guage indicated an IMMEDIATE drop > in boost. Right you are. A turbocharger is driven by a turbine that, in turn, is driven by exhaust gases. If you let up on the throttle, that decreases exhaust pressure, which decreases turbine torque, which decreases boost. This phenomenon is a great source of extra complexity in turbocharged airplanes, where, for example, if you put the manifold pressure at the redline at the beginning of the takeoff roll, you may blow your engine...
marc@haddock.UUCP (01/03/86)
Someone please correct me if I am wrong, but I thought that the method of operation behind a turbo was as follows: There is a dual bladed fan with some kind of a shaft between the two blades. One side of the fan is inside of the exhaust manifold and is rotated by the exhaust of the engine. The other side is connected to the air intake of the carb. Therefore, when you accelerate, the exhaust spins the fan faster producing a higher flow of air into the carb which eventually makes it into the cylinder chamber. If you let off on the throttle, less exhaust is produced and therefore less boost presure. -- Marc Evans - WB1GRH Interactive Systems Corp. Boston, MA
eric@amc.UUCP (Eric McRae) (01/03/86)
In article <2@emacs.UUCP> pz@emacs.UUCP (Paul Czarnecki) writes: >Take the case where you are charging down the road at 4,000 rpm and you >let up on the throttle without depressing the clutch. Under the above >theory the turbo would continue spinning at tc(4000) since the engine is >still at 4,000 rpm. However the boost guage indicated an IMMEDIATE drop >in boost.... why is this so? The boost guage usually measures relative pressure/vacuum on the engine side of the throttle. The turbocharger is on the other side. What you're seeing is the results of an engine that was breathing freely but just had its air supply cut off. If you stuck a pressure guage on the compressor side of the throttle, you would see continued high pressure as you expected. (Well "continued" for a fraction of a second or so anyway. Regarding your tc(x) theory: Remember that the power for the compressor is supplied by expanding exhaust gasses. An engine may be turning at 4Krpm but if the throttle is closed, there is litle in the way of exhaust gas to power the compressor. At that point, the significant energy source for the compressor is its rotational inertia. Most turbochargers are designed to have a minimum amount of inertia. Turbocharger output is a function of the power being generated by the engine. Thus, the power available from a turbo on a car that is doing a steady 55 mph on a steep grade is much more than that from a car doing a steady 55 on a level road. Turbo lag is caused by a combination of the rotational inertia of the turbocharger and the compressability of the air between the turbocharger and the engine and the exhaust gasses between the engine and the turbocharger. Finally, turbocharged engines are positive feedback systems. A running engine delivers power to the turbocharger which pushes more air at into the engine which exhausts more gas at the turbocharger which.... The negative feedback element is the wastegate, which causes exhaust gasses to bypass the compressor when the pressure at the engine reaches a safe maximum. If your wastegate fails (stuck closed) you can blow up your engine in a few seconds. BTW, I own an intercooled Volvo turbo wagon. I blew a pipe off the compressor one day and can attest to the continued high pressure after you let up on the throttle. I and the fellow that I was passing at the time, thought I had a rocket under the hood from the sound of the escaping air :-).
mike@amdcad.UUCP (Mike Parker) (01/05/86)
In article <4766@alice.UUCP> ark@alice.UucP (Andrew Koenig) writes: >> Take the case where you are charging down the road at 4,000 rpm and you >> let up on the throttle without depressing the clutch. Under the above >> theory the turbo would continue spinning at tc(4000) since the engine is >> still at 4,000 rpm. However the boost guage indicated an IMMEDIATE drop >> in boost. > >Right you are. A turbocharger is driven by a turbine that, in turn, >is driven by exhaust gases. If you let up on the throttle, that >decreases exhaust pressure, which decreases turbine torque, which >decreases boost. > Maybe the decrease in exhaust gases is a contributor, but I believe it is a small one. Turbo lag is mostly caused by rotational inertia of the impeller, and this inertia should keep it spinning a least for a second when the throttle is let up. In my car the "boost gauge" is not a boost gauge at all, it is a manifold pressure gauge. When I lift the throttle, the turbo may still be pushing hard, but with the throttle plate closed it doesn't get to the manifold. I imagine that a gauge measuring the pressure differential between the atmosphere and the turbo output would go up suddenly when the throttle closes. Mike
pz@emacs.UUCP (Paul Czarnecki) (01/06/86)
<fnord> aha! of course it is the exhaust gasses that spin the silly little thing. I assumed (incorrectly that) that high rpms meant high gasses. wrong! Thanks to all those who responded to me: gcc-milo!zrm (Zigurd R. Mednieks) rti-sel!amr (Alan Roberts) seismo!USC-ISIB.ARPA!JDEIFIK (Jeff Deifik) moncol!ben (Ben Broder) abnji!saab (dennis) amdcad!phil (Phil Ngai) (Jeff, it pz, not ps, lucky Postmaster was in a good mood that day...) So the moral of the story is... If you want more boost, you need a large amount of exhaust gases which are cutoff by closing the throttle so therefore... keep on chargin! (don't let up on the throttle, EVER! :-) > (super charger trivia -- back in the 50's a lot of MG owners put > superchargers on their cars. Name the city that made (most) of these > and name at least one other metal sporting good part that this city > made. hint: it's near my hometown :-) Conshohocken, Pennsyvania. Thay also make aluminum baseball bats. I read about super chargers in a R&T collecters classic on MG a few years back. I know abouthte bats because I've held one in my hand. (I won't claim to have hit a ball with it ever, I tried...) pZ -- Just in time to be too late. Paul Czarnecki Uniworks, Inc. decvax!{cca,wanginst!infinet}!emacs!pz 20 William Street emacs!pz@cca-unix.ARPA Wellesley, MA 02181 (617) 235-2600 -- -- My wife ran off with my best friend, and I miss him. Paul Czarnecki Uniworks, Inc. decvax!{cca,wanginst!infinet}!emacs!pz 20 William Street emacs!pz@cca-unix.ARPA Wellesley, MA 02181 (617) 235-2600
davidsen@steinmetz.UUCP (Davidsen) (01/06/86)
In article <7974@amdcad.UUCP> mike@amdcad.UUCP (Mike Parker) writes: >In article <4766@alice.UUCP> ark@alice.UucP (Andrew Koenig) writes: >>> Take the case where you are charging down the road at 4,000 rpm and you >>> let up on the throttle without depressing the clutch. Under the above >>> theory the turbo would continue spinning at tc(4000) since the engine is >>> still at 4,000 rpm. However the boost guage indicated an IMMEDIATE drop >>> in boost. >In my car the "boost gauge" is not a boost gauge at all, it is a manifold >pressure gauge. When I lift the throttle, the turbo may still be pushing >hard, but with the throttle plate closed it doesn't get to the manifold. Actually the turbo is not "pushing hard". The throttle plate is upstream (airstream) of the turbo, while the manifold is downstream. When the throttle plate is closed there is a vacuum on the input side of the turbo, resulting in no air to be compressed into the manifold. Also note that while turbo lag is caused by the time to get the turbo up to rotational speed, there is also a lag caused by the distance from the throttle plate to the input valves. An example of this could be seen when the Paxton blowers used on mid-50's Studebakers (and a very few 57 Fords) were moved to other cars. Traditionally the carburator is placed before the blower, and a notable lack of throttle response is caused. The Studebaker setup pressurized the whole carb in an air box, keeping the throttle plates close to the valves. This resulted is much better drivability around town. We also played with doing this with a GMC 6-71 blower and handmade manifold once, and got the same results. NOTE: if you try this, all carb vents *MUST* be under pressure or they become a sprinkler system. Also a "popoff" valve is needed between the blower and the carb, so that closing the throttle at high rpm won't cause very high pressures. The high pressures will blow not only the obvious things, but can bend the throttle plates (calculate 75psi x area), which results in a permanent half throttle effect. This can be a real thrill. I have had a chance to drive a number of turbo'd cars in the last few years, and have yet to find one that had the same responsiveness as a positive displacement blower. In addition, having the turbo "come on" while cornering at high speed and throttle can cause control problems. I don't deny that they are cheaper to install and run than a blower, but if I had a choice I would rather have a blower for street use. -- -billd seismo!rochester!steinmetz! unirot / \ / ihnp4! crdos1!davidsen \ / chinet! -----------------/ "It seemed like a good idea at the time..."
ems@amdahl.UUCP (ems) (01/07/86)
The reason boost is not related only to engine rpm and
time is that it is the VOLUME of exhaust (and hence
it's velocity, temp and pressure - but I'm supposed
to be simplifying here for illustration ...) that you
care about. Less throtle, less exhaust gas, less turbo boost;
even if rpm is constant.
--
E. Michael Smith ...!{hplabs,ihnp4,amd,nsc}!amdahl!ems
This is the obligatory disclaimer of everything.neal@weitek.UUCP (Neal Bedard) (01/08/86)
In article <2493@amdahl.UUCP>, ems@amdahl.UUCP (ems) writes: > > The reason boost is not related only to engine rpm and > time is that it is the VOLUME of exhaust (and hence > it's velocity, temp and pressure - but I'm supposed > to be simplifying here for illustration ...) that you > care about. Less throtle, less exhaust gas, less turbo boost; > even if rpm is constant. [First, remember that Saab uses the `blow-though' turbo setup.] The original poster alluded to the fact that there is quite a bit energy left in the rotating turbine/compressor immediately after the throttle is closed. Thus, the pressure on the *upstream* side of the throttle butterfly may remain quite high until the turbo has dissipated its stored rotational energy by raising the pressure (and temperature) of the air *upstream* of the throttle. The boost gauge plunges to zero at drop-thottle because the boost pressure is measured *downstream* of the throttle butterfly in the intake manifold (the only meaningful place to measure boost pressure.) The intake manifold will be in *vacuum* until the throttle is re-opened and the turbo spins up again. -Neal -- UUCP: {turtlevax, resonex, cae780}!weitek!neal