Engine Braking

[Miller88]

Originally Posted by Arragonis

Is this why people say to maximise the the mythical LOD thing on the SG2 which I still don't understand ? In which case I regard this posting as a potential magic key. I hope.




In what way - they stop sooner, no braking or no fuel shut off ? I noticed my TDI would shut off at anything over idle at all, the Aygo Petrol has to be over 1400 when you start engine braking for the ECU to detect it.

Most manual transmission vehicles i've driven with DBW throttles will not slow themselves down ... they just keep going.

the only way I can get my Focus to slow down without using the brakes, is to set the cruise control and hit the decelerate button on it - that shuts the throttle and lets the car slow down.

My focus cuts the injectors as DFCO, but does not shut the throttle

[wobombat]

Manifold restrictions in gasoline engines restrict the flow of air into the cylinders. The lower mass of air means lower compression in the cylinder which means much less power produced for the same amount of fuel consumed. This is because higher compression is a better "lever" to produce power. An engine produces power by compressing air and fuel then igniting the fuel, which expands the combustive mixture which produces pressure to push the piston and make power. If you reduce the in cylinder compression then you have much less power produced.

Why is it not then the case that the point of highest torque is the point of highest efficiency? I was discussing this with someone who thought this to be the case, and I found myself unable to effectively refute that except by saying that high rpm's are unefficient. I know that torque is technically ft.-lbs. but I have always thought of it as ft.-lbs per stroke of the engine. If torque is indeed the amount of work done per stroke of the engine, then the point of highest torque would be the point of highest compression ratio, and probably then the point of highest efficiency.

[serialk11r]

Originally Posted by Miller88

Most manual transmission vehicles i've driven with DBW throttles will not slow themselves down ... they just keep going.

the only way I can get my Focus to slow down without using the brakes, is to set the cruise control and hit the decelerate button on it - that shuts the throttle and lets the car slow down.

My focus cuts the injectors as DFCO, but does not shut the throttle

Is this right? I always thought that wasn't the case... I don't think NOx increases when you shut the throttle plate because nothing is being burned so there's no high temperature for NOx to form. However, leaving the throttle open under deceleration means that you're pumping cold air through the cats which cools them down, and that could be bad.

[kennybobby]

Originally Posted by wobombat

Why is it not then the case that the point of highest torque is the point of highest efficiency?

i think you answered your own question.

The peak torque that an engine generates does occur at the point of highest volumetric efficiency--when the airflow has the least restriction and the most power is made with the least amount of fuel. The engine speed that this occurs is the ratio of the power:torque.

Engines don't make speed, they make torque. Speed is what results when the load torques equal the generated torque from burning a certain amount of fuel.

Plot torque on the x-axis and speed on the y-axis of a BSFC chart with arcs of constant power to see this graphically.

[Arragonis]

Originally Posted by Miller88

Most manual transmission vehicles i've driven with DBW throttles will not slow themselves down ... they just keep going.

the only way I can get my Focus to slow down without using the brakes, is to set the cruise control and hit the decelerate button on it - that shuts the throttle and lets the car slow down.

My focus cuts the injectors as DFCO, but does not shut the throttle

Interesting, I haven't driven a DBW petrol engine (with a genuine throttle) to try this out - George is an old fashioned Fly By Pants throttle and is the first Petrol I've driven since 2002.

The TDIs would just shut off fuel because the demanded fuel was below the fuel needed for the engine to maintain its current speed - i.e. (for the ECU) it was on overrun.

[wobombat]

I don't think the point of highest torque always lines up with the point of highest efficiency as seen on BSFC charts. For example, my brother's civic has a peak torque at 7000 RPMs. My car has a peak torque at 4750RPMs. Yet the common knowledge around here seems to be that operating your car over about 3000 rpms tends to be very fuel inefficient. What could account for the difference, or is there actually no difference and these car models really do produce power the most efficiently at these numbers.

[serialk11r]

Originally Posted by wobombat

I don't think the point of highest torque always lines up with the point of highest efficiency as seen on BSFC charts. For example, my brother's civic has a peak torque at 7000 RPMs. My car has a peak torque at 4750RPMs. Yet the common knowledge around here seems to be that operating your car over about 3000 rpms tends to be very fuel inefficient. What could account for the difference, or is there actually no difference and these car models really do produce power the most efficiently at these numbers.

Peak efficiency is pretty much never at peak torque since peak torque is when your volumetric efficiency is highest. More air, but also proportionally more fuel injected. Highest volumetric efficiency can be achieved at any point in the rpm range with cams and manifold/header tuning.

Peak thermal efficiency rpm is where lower cooling losses from running the engine faster balances out the increased friction and parasitic loss from running the engine faster. For "Otto cycle" engines (non-"Atkinson cycle") the gain in power from increasing volumetric efficiency/load doesn't balance out the increased amount of wasted pressure not captured by the expansion stroke that is simply blown out the exhaust (aka, more heat energy ending up in the exhaust).

[bestclimb]

[QUOTE=Miller88;331536]Newer vehicles don't seem to engine brake as well. Vacuum on deceleration = NoX emissions.[/QUOTE

I have not found this to be true. every newer vehicle I have driven engine brakes great, even the autos if you tell it to down shift.

[redpoint5]

Originally Posted by Miller88

Most manual transmission vehicles i've driven with DBW throttles will not slow themselves down ... they just keep going.

My worthless anecdotal evidence is my TSX. It's a DBW manual, and it does DFCO braking just fine down to about 1,000 RPM.

[serialk11r]

Okay today as I was coasting down to a toll booth I tested cutting the ignition and putting my foot all the way down (I have cable throttle) to reduce pumping. Braking effect was not noticably decreased at 3000rpm (but hard to tell from seat of pants dyno). So I think the charts I've seen on the internet are probably about right, friction is on the order of 100-150 kPa mean effective pressure, pumping is at most maybe 70.