Crazyquik

Recently I got into a discussion with someone about engines under load.

They made the comment that during engine braking, you dont have combustion taking place. I didn't really believe this, and they went on to state that whenever the engine in a modern fuel injected car is in a no load state (idle, neutral, whenever your foot isn't on the gas, etc) that no fuel is being combusted.

Now, its my thinking that as long as an air pump (internal combustion engine) is running and turning rpms, there should be combustion taking place, right? I mean, I still hear the exhaust, and can feel the pulse of the byproducts coming out the exhaust pipe. Also, the tach at idle (no load) is bumping around and showing rpm.

When I raised these concerns with him he gave me the "well I'm an engineer and thats the way it is" thing.

So, is there combustion taking place in no load situations, idle, coasting in neutral, etc? What would make him say what he did?

J.

2xLS1

iTrader: (25)

He must be a train engineer then. There is combustion taking place anytime an engine is running period.

Fireball

iTrader: (21)

that engineer must've failed thermodynamics
I too am an engineer (and I know better) LOL

combustion always occurs right before the power stroke in any 4-cycle engine...regardless of load

BIG PD

if there was no combustion the damn piston would stop moving...hence the motor would stop running.

30thWS6

Ok trust on this one, there are plenty of moronic engineers out there. So just because they say they're engineers doesn't mean they know anything about anything. Your friend was partly right however, yet grossly mistaken. During deceleration coasting etc., the engine management system on some vehicles goes into a state of fuel cut-off. So during this period no fuel is being combusted because there isn't any fuel being injected. Now this is only on some vehicles, and never at idle, or any other condition other than decel and coast. So tell your engineer buddy to stop giving engineers such a bad name.

Fireball

iTrader: (21)

yes this is possible...but definately not the case in non-fuel injected cars. Carburators put in fuel based on airflow thru them. on decel the throttle blades are closed but the carburator is still adding some fuel

30thWS6

"I didn't really believe this, and they went on to state that whenever the engine in a modern fuel injected car is in a no load state (idle, neutral, whenever your foot isn't on the gas, etc) that no fuel is being combusted."

Even the most advanced of the feedback carburetors had very rudimentary engine management systems. So, in no way was I implying that this applied to any carbureted vehicle.

99 Black Bird T/A

iTrader: (4)

He's an engineer and he should be willing to prove his statements...asking him to fill up his gas tank and then leave his car/truck/suv running in neutral/idle in park or whatever all day and see if his gas gauge reads differently after say eight hours...

Skelton

Yes if its a diesel. Some do cut the fuel off hence no combustion untill a lower rpm is reached. Some diesels are hard to decel due to mechanical compression ratio. Otherwise you have combustion, just not enough to overcome decel due to load.

30thWS6

Gas engines also use fuel cut off on decel. Where did you get this information that only diesels use this strategy.

Bimmer Tech

iTrader: (1)

You guys are on the right track, but are mixing up a few terms. The engine has to have fuel to idle, no question about it. Weather it be sitting in your driveway or coasting down the interstate in neutral, its combusting the air/fuel mixture set by the idle circuits in the computer. This is also an open loop operation. Engine braking is a different situation. Some engines (i'm not sure about the LS1...yet) will turn off the injectors when engine braking is present. This is because there is NO load on the engine. This is also an open loop operation since the O2s go to near zero voltage (lean) reading. I've seen this several times while monitoring injector on time and O2 signals during testdrives. Hope this helps.

-Chuck T.
8balls@crackdealer.com

StevieZ

iTrader: (3)

The ECU in the Dodge Viper shuts the injectors on decel on any RPM above 2200