29 replies to this topic

[R1minimagic]

I have another fuel saving idea....a start/stop system.

When you come to a halt you turn the engine off with the key and when you want to move you start the engine again with the same key, simples!!

[mk3 Cooper S]

OVERUN FUEL CUT OFF is not as simple as just cutting the fuel.
It relies on certain inputs.
Usually mapped against throttle position/engine speed/vehicle speed/gear.
This will give you calibrated enable/disable conditions.

Also the ign advance needs to be calibrated on entry and exit to avoid push on on fuel on and high H/C and popping in the exhaust on fuel off.
Lots of work for little gain especially when a drive cycle emissions test does not need to be passed.

Edited by mk3 Cooper S, 10 May 2012 - 01:48 PM.

[Ethel]

The way Rover/SU did it was to block the air vent to the float chamber so the venturi can't create the pressure differential to suck the fuel through. Hif E's (Montegos etc) and Kif (K series engined Metros) carbs have a solenoid valve built in, but you could probably rig up similar on a HS with a piped vent on the float lid.

[mk3 Cooper S]

The way Rover/SU did it was to block the air vent to the float chamber so the venturi can't create the pressure differential to suck the fuel through. Hif E's (Montegos etc) and Kif (K series engined Metros) carbs have a solenoid valve built in, but you could probably rig up similar on a HS with a piped vent on the float lid.

A pound to a pinch of ******* that it will stall on a clutch depression if you just impliment that

[R1minimagic]

OVERUN FUEL CUT OFF is not as simple as just cutting the fuel.
It relies on certain inputs.
Usually mapped against throttle position/engine speed/vehicle speed/gear.
This will give you calibrated enable/disable conditions.

Also the ign advance needs to be calibrated on entry and exit to avoid push on on fuel on and high H/C and popping in the exhaust on fuel off.
Lots of work for little gain especially when a drive cycle emissions test does not need to be passed.

Exactly, this is the only reason the OEMs did it as CO2 emissions were not legislated back then

[Ethel]

Must have worked well enough to go into production and be developed for the K Series.

The advantage is it leaves the float chamber full so fuel can flow as soon as the vent opens, it would be quicker than restarting a warm engine. Setting the operating rpm far enough above idle speed ought to get round the declutching problem. Well worth a go with MJ, all you need is a solenoid valve.

I think Rover did it as they were lagging in adopting fuel injection. As you say it's only going to operate under limited conditions when not much fuel will be used anyway, but it's very simple to implement and shouldn't be problematic, or damaging, as the engine will be running well below optimum anyway. If it's setup so it reverts to normal enough before the point where you need to declutch it would be unnoticeable to the driver, save for perhaps minimal lag if you boot it from overrun. How much time would you spend in overrun if your commute is full of traffic lights and stop start driving I wonder?

[mk3 Cooper S]

Must have worked well enough to go into production and be developed for the K Series.

The advantage is it leaves the float chamber full so fuel can flow as soon as the vent opens, it would be quicker than restarting a warm engine. Setting the operating rpm far enough above idle speed ought to get round the declutching problem. Well worth a go with MJ, all you need is a solenoid valve.

The stepper motor or extra valve will also deliver more air in that condition and a rake load of retard

[Ethel]

Don't follow your reasoning, there's no effective airflow in the float vent, it only communicates pressure, usually atmospheric, so you can create a pressure differential across the venturi. If anything, opening the vent might allow a tiny bit of inertial ramming from the air it lets in which would help in restoring fuel flow.

To get a true angle on this we should look at petrol density and what pressure differential supports what column of it in a SU float chamber & jet. A 1mm diameter jet must give a pretty big ratio compared to the area of the float chamber, several thousands to one? - Even without a needle sat in it. I reckon the holding the fuel the odd mm down the jet would stop the flow and need minimal pressure.

[mk3 Cooper S]

If you cut fuel to the carb and dont increase the aiflow you are effectivly turning over the engine using the rotation/inertia of the drivetrain but if you dont increase the air though the engine it will decelerate very quickly. Therfore before the fuel can be reinstaled and burnt the engine will have stalled.

With the idle/dashpot setting of a HIF 44 there will not be enough air going through the engine to provide a decent deceleration rate.

[Ethel]

In overrun the engine is being driven by the car via the gearbox. Putting the clutch in will allow the engine to slow but it will reduce the manifold vacuum as it does, so you could restore the fuel supply before it got to idling vacuum/speed. I doubt the combusted fuel is contributing much, if anything, but you could just turn off the ignition to see*. I actually suspect the chambers will be too empty for effective combustion, another reason for fuel cut off if you have to have a catalytic converter - a consideration for Rover?


* if cutting fuel slows you faster then it sounds beneficial, if anything, for the performance enthusiast. You'd have more control with the throttle alone.

[R1minimagic]

The combusted fuel contributes very significantly to the flow of gas through an engine as it enters as a liquid and exits (mainly as CO2 and water vapour) as a gas, and 1 mole of gas occupies 22.4 litres at STP. 1 mole of fuel also produces many moles of CO2 and H2O.

On my golf TDi if you try left foot braking with right foot still on throttle it gives a massive braking effect vs normal braking so seems like it is cutting the fuel on braking only

[klivins]

... but you could just turn off the ignition to see...

I doubt it. Yes, the combustion will not happen but because there will be no spark. The fuel from the carb will enter the cylinders and will be partly drawn out into exaust where it will create nice effect when combustion occurs!

All in all I think I would give it a try if will accidentally find suitable valve.
Or I did not understood what you said?

[R1minimagic]

Also, remember that any engine needs a certain minimum cranking speed in order to start so if you are going too slow when you put your foot back on the throttle it won't start, you will need to use the key/starter motor!

[Ethel]

I only suggested turning off the ignition to simulate any effect cutting off the fuel on overrun would have on engine braking. It would be a bad idea to do it as routine, and pointless as the turning engine will still draw fuel out of the carb. An engine turning slower than idle speed is likely to stall in any case, but it wouldn't develop enough manifold vacuum to trigger our overrun cut off solenoid.

[ian2000t]

I tried this yesterday on my drive home - and going downhill in third at 30mph, turning the ignition off to simulate a fuel cut caused no decceleration at all. Yes, when I switched it back on there was a bang in the exhaust (don't try this at home kids) which isn't going to do any harm for one test before anyone jumps!

SO, I still think if it is possible to somehow stop the fuel supply into the engine on overrun, it would be beneficial.