Race Engines: Why no fuel cut during over run
Race Engines: Why no fuel cut during over run
(OP)
I first really noticed this when I went to see my first grand prix and since then I've noticed that many fuel injected race cars pop and backfire like crazy on the over run. I'm referring to naturally aspirated cars, not turbo charged cars which are often equipped with anti lag systems that causes popping on the over run. So what is the benefit to injecting fuel during the over run? During the traction control era in F1, I figured that maybe the ECU fired certain cylinders during braking in order to prevent rear wheel lock up, but that should be illegal now and yet I still hear them on TV popping during braking.





RE: Race Engines: Why no fuel cut during over run
A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare
www.sixstroke.com
RE: Race Engines: Why no fuel cut during over run
In order to reduce emissions, the decel fuel cut threshold has to be assuredly above the engine "load" (or negative load as the case may be) that results in misfiring. This results in a noticeable abruptness in the throttle response at slightly-cracked throttle. You go into a corner on the brakes (fuel is cut off), try to smoothly apply throttle on the exit, at first the engine does nothing then "whack" the fuel comes on and the engine takes up the load. This is not much fun when you are on a bike cornering at the limit of traction, particularly when the bike weighs 400 lbs and has 120 - 160 horsepower - driveability is extremely important. Some models are worse than others for this; I rode a stock 2003 Yamaha R6 and did three laps then brought it in ... horrible driveability. My stock ZX10R has decel fuel cut, but it's below the misfire threshold and is scarcely a bother. I suppose they just let the catalytic converter deal with it.
If you want to fix this, you have to keep injecting fuel down to a smaller "load" threshold. This means there will be some misfiring, but it's better to have that, than to have an abrupt "on/off" behaviour. The misfiring means there will be some popping out the exhaust.
In the bike applications, the piston and exhaust valve temperature isn't a big enough issue to warrant injecting gross amounts of fuel on deceleration, but that doesn't mean it isn't an issue in other applications.
Fuel consumption is an important issue in MotoGP because of the restricted fuel tank size, but those engines are using drive-by-wire throttles and are deliberately opening throttles on deceleration to cut back on engine braking. I don't know if they are cutting off fuel simultaneous with opening up the throttles, but it's quite possible with the way engine management controls are going these days.
RE: Race Engines: Why no fuel cut during over run
- Steve
RE: Race Engines: Why no fuel cut during over run
This is not a problem of EFI per se but more a symptom motorcycle EMS being about a zillion years behind that of cars - although they are rapidly catching up!
MS
RE: Race Engines: Why no fuel cut during over run
Anyone know the strategies used by manufacturers in their ECUs.
RE: Race Engines: Why no fuel cut during over run
You can trick some of them by telling it that the throttle is a bit more open than it actually is, but that has other side-effects.
RE: Race Engines: Why no fuel cut during over run
When deceleration fuel cut is on and throttle is opened engine should catch easily and not with abrupt power. If it does then the ecu tuning is not good.
RE: Race Engines: Why no fuel cut during over run
RE: Race Engines: Why no fuel cut during over run
As such, I've gone as far as installing an ignition cut on shifting for a nice 2 foot white flame (and LOUD report) from the tailpipe on every shift. Races are by invitation and I want to make sure that I'm invited!
RE: Race Engines: Why no fuel cut during over run
A guy who retired from Siemens, now continental, figured out how to develop a solenoid injector with over 22:1 turndown using conventional injector driver electronics to accomodate the trend towards oem's desire to maintain closed loop during decel for optimum catalyst storage. I ran a cng fueled vehicle and he ran a gasoline vehicle supplied by chrysler just for this activity. The injector with its own electronics molded internally.. Management killed it. Now BMW uses scaled injector pressure to bridge the gap but high linearity injectors are the real answer and if the manufacturers had some gonads they would promote what is now an 11 year old field tested idea..
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RE: Race Engines: Why no fuel cut during over run
RE: Race Engines: Why no fuel cut during over run
Several clients ran into calibration snafu's when cylinder to cylinder a/f strayed too far.. Saw over 10% var after hot shut down restarts and some other events.
T