Fuel Consumption: Explanation Please

[imported_Dodgy]

This refers to the "instantaneous" fuel consumption read out on instrument panel.

SCENARIO:
Cruising along (easiest downhill), take note of consumption.

Take foot off the accelerator, and consumption drops to 0.0 L/100k relatively quickly (pretty cool). RPM's above idle.

Without doing anything else, depress the clutch, & consumption GOES UP (to 3-4 L/100km). RPM's drop to idle.

I am sure there is some wonderful technical explanation for this, but I simplistically assume that higher RPM's translate to higher fuel consumption

Could anyone enlighten me?

[Citizen Z]

When the engine is breaking the car (coasting in gear), the injectors are not providing any fuel. Depress the clutch and the engine drops to idle, using the idle level of fuel. You can be going fast enough that even though you've depressed the clutch, you're still going to get seeminly infinite efficiency but thats because your speed/consumption is more than 99.9mpg (or whatever the L/100km equivalent is.

The car manual says that when coasting in gear, the car doesn't use any fuel.

[imported_Dodgy]

Originally Posted by Citizen Z

The car manual says that when coasting in gear, the car doesn't use any fuel.

Amazing, but, and I'm going to sound dumb here, how to the cylinder chambers cope with no fuel as part of their cylce. Granted, when combusting, I guess the main component of the chamber is air.

Doesn't the motion of the pistons cause the air to expand / contract with change in volume? Would the pressure changes be an issue? (Probably not?). Do the valves (inlet/outlet) stop opening?

I think I have trouble conceptualising the cylinders being "empty" (ie just air), but I am sure I am being, as I said, a bit dumb.

[plien69]

Everything still happens the same: cylinders go up and down, valves open and close. Just no fuel is delivered and no spark is ignited.

Yes, air is still compressed in the cylinder, and it is this effect that provides the engine braking force. The engine is able to keep turning due to the momentum of the car.

Once you press the clutch pedal, you decouple the car's momentum from the engine, and thus the DME has to keep the engine turning by introducing fuel and driving the engine at idle speed.

[1Ronnie]

As an aside to this, my brother-in-law who is a mechanic, and has designed some of his own engines, suggests that during the 4 strokes when engine braking, there is both compression of air on the compression stroke, but also vacuum during the explosion downward stroke.

During early breaking before the rings have sealed well, engine breaking can draw oil up the side of the cylinders which can be bad for the engine. He strongly recommends avoiding engine breaking during break-in.

[plien69]

Originally Posted by arrons1969

As an aside to this, my brother-in-law who is a mechanic, and has designed some of his own engines, suggests that during the 4 strokes when engine braking, there is both compression of air on the compression stroke, but also vacuum during the explosion downward stroke.

During early breaking before the rings have sealed well, engine breaking can draw oil up the side of the cylinders which can be bad for the engine. He strongly recommends avoiding engine breaking during break-in.

Interesting. I have heard the opposite: that it's important to include engine breaking during break-in, in order to make sure the engine experiences loads in the "opposite direction" as normal acceleration.

[scs]

Originally Posted by plien69

Interesting. I have heard the opposite: that it's important to include engine breaking during break-in, in order to make sure the engine experiences loads in the "opposite direction" as normal acceleration.

yup, cylinder back pressure is a good thing, it forces the rings to expand outward which in turn evenly seats them to the cylinder walls.