Engine and Transmission question

[brockley]

My friend and I got into a discussion (read arguement) last night about engines and transmissions while playing Gran Turismo. He says he knows a lot about cars and says that when you let off the gas, the fuel injectors stop spraying fuel into the combustion chamber. He also says the same for when the transmission shifts. I know this is false for manual transmissions because when you depress the clutch, it disengages that from the flywheel which is what makes the transmission spin. I know from experience that when you depress the clutch, you can rev as high as you want which wouldnt be possible if the fuel injectors stoppped working while the clutch was depressed. I was also under the understanding that an automatic worked the same way except that the clutch separation was automatic as the name implies. My main question is this, do the fuel injectors stop spryaing fuel into the combustion chamber when you let of the gas or shift (manual or automatic)? It just seems like the ingine would stop turning if there was no combustion to send the pistons back down after compression. Maybe the fuel injectors spray less fuel but they do not shut off completely or your car would die each time you stopped at a stop light and let go of the gas pedal like most people do.
Thanks
brockley

[Kuan]

In most states it's illegal to allow a car to coast without the engine running.

[brockley]

I wasnt really questioning the legality of it, just what he was telling me. "When you shift you let of the gas." I dont know about you but in an automatic, I never let off the gas when accelerating to allow the car to shift. I also knew he was bsing when he said there is no fuel coming out of the injectors when you are shifting. Oh well, guess it was good i beat his ass anyway.
brockley

[Eberhard Weilke]

Guys, serious...

Any Mercedes with K-Jetronic after, I think, MJ 82, has an automatic fuel shutoff when the engine is coasting.

When you have more than 1500 rpm (guess why there is a sensor, it is not because of the cruise control) and lift of the foot of the trottle a vaccum engaged device shuts of the fuel supply to the injection nozzles.

Concerning the automatic transmission: Newer cars with databus between gearbox and engine control in fact reduces power output by shuting of the fuel supply during shifting of he tranny to avoid harsh shifting.

Kind regards
Eberhard
(who sometimes wonders, why some people have the convidence to give answers when in fact they have no clue what they are talking about)

[Eberhard Weilke]

BTW: The fuel at gasoline engines is never sprayed into to combustion chamber, but into the air intake manifold.

Even at the old style Mercedes Diesel (OM 621 ff., OM 601 ff.) always sprayes into the pre chamber.

Direkt injection used to be a technology for certain truck engines, for the new generation of tdi-engines of Audi and VW and for the cdi-engines of new Mercedes. And, yes, there used to be some Rover and Fiat with direct injected Diesels around.

There are some direct injected gasoline engines. First was Mitsubishi with the gdi-engines, today VW and Audi has the new technology on the market. And Mercedes is also introducing new engines with this technology.

Well, look back in history: The 300 SL Gullwing and the Gutbrod Superior had also direct injected gasoline engines. The guy who designed them did his training at the Luftwaffe during WW II, where direct injection was commonplace for fighter plane piston engines.

Kind regards
Eberhard
(living pretty much in the middle between the place where the Gullwing was developed and where Gutbrod had their plant)

[brockley]

So now youre saying there is no combustion going on when costing or changing gears? Youre confusing me. Can one person explain what happens when changing gears as far as the engine is concerned?
Thanks.

[engatwork]

He may be right if he is talking about that Gran Turismo car.

[Eberhard Weilke]

There is no combustion going on when e.g. you release the accelerator coasting towards a red traffic light and the engine is pushed by the momentum of the car above the 1500 rpm mark.

Also when descending a hill and you keep the car into gear the vaccum valves shuts the fuel of --> no combustion.

This is the story for all K-Jet engines you find in W 123, W 126 R 107 and alike after MJ 82 (or so)

The feature of the power laps during shifts of automatic transmissions only applys to cars with networked computer control of engine and gear box like in new Lexus, new Mercedes and alike.

The W 123 and W 126 (and early 124 with four speed auto) is vaccuum operated and has no influence on fuel supply.

Kind regards
Eberhard

[brockley]

Ok thanks. This information is for Mercedes only though right? What about some of the systems used in the LeMans prototype cars or are those the same basic system?
Thanks.

[Eberhard Weilke]

The Bosch K-Jet was pretty common in many other cars like VW Rabbit, Quantum, Scirocco, the fleet of Audis 4000 and 5000, Porsche.

Similar systems you find at BMW and others.

I guess that pretty much every fuel injected gasoline car in Europe by the end of the eighties had this fuel shut-off device.

Don't know much about the US cars though.

Kind regards
Eberhard