Nick M Posted July 20, 2005 Share Posted July 20, 2005 As I see it, a sudden loss of load will merely be like revving the engine with no load - and the rev limiter should stop it going any higher. I cannot see how the engine would rev any quicker/higher just because it was under load one second, and free revving the next... A very bad comparison (because of the speed that you go from one state to the next, compared to a sudden change), but when you're driving down the road with the engine pulling hard while accelerating and you dip the clutch, what happens ? The revs rocket upwards *VERY* quickly. If you're near the top of the rev range and the engine is under heavy load then the ECU really has it's work cut out to deal with the inertia since it can't mechanically slow the crank with a brake, it has to do it by limiting the fuel / spark. That can take a brief period to have any effect. Seems to be borne out by the comments about the busa engines. Quote Link to comment Share on other sites More sharing options...
stu999 Posted July 20, 2005 Share Posted July 20, 2005 A very bad comparison (because of the speed that you go from one state to the next, compared to a sudden change), but when you're driving down the road with the engine pulling hard while accelerating and you dip the clutch, what happens ? The revs rocket upwards *VERY* quickly. Nick, I am just having trouble picturing where the extra energy comes from to accelerate the engine quicker if drive was disengaged when comparing it to free revving... The inertia of the rotating components will keep the engine turning (the flywheel etc), but in both cases, to accelerate rotating components from X rpm to Y rpm is going to consume the same amount of energy. I need to understand where the 'reserve' is for my brian to comrehend what is going on here... Yes, the engine is likely to accelerate a tad slower if it was held at lets says 6000rpm 'free state' and then floored to 9000rpm, than holding it flat out under load at the same revs then dropping the clutch pedal to the floor. But would there be any difference if the time could be measured between 6000 and 9000 if the throttle was buried to the floor from idle? Quote Link to comment Share on other sites More sharing options...
Lurksalot Posted July 20, 2005 Share Posted July 20, 2005 I need to understand where the 'reserve' is for my brian to comrehend what is going on here... I have a bit of confusion on this topic , I see you do , but I thought your Brian might understand Quote Link to comment Share on other sites More sharing options...
Nick M Posted July 20, 2005 Share Posted July 20, 2005 Nick, I am just having trouble picturing where the extra energy comes from to accelerate the engine quicker if drive was disengaged when comparing it to free revving... The inertia of the rotating components will keep the engine turning (the flywheel etc), but in both cases, to accelerate rotating components from X rpm to Y rpm is going to consume the same amount of energy. I need to understand where the 'reserve' is for my brian to comrehend what is going on here... Yes, the engine is likely to accelerate a tad slower if it was held at lets says 6000rpm 'free state' and then floored to 9000rpm, than holding it flat out under load at the same revs then dropping the clutch pedal to the floor. But would there be any difference if the time could be measured between 6000 and 9000 if the throttle was buried to the floor from idle? Errrr, the transmission.... If you suddenly remove the "drag" on the engine caused by the transmission there's suddenly a lot more energy which needs to go somewhere. Try this analogy - take a small scalextric motor, connect it up and let it spin. Now hold the spindle between your fingers and pretend you're the gearbox, diff, wheels, tyres - the motor has to work against the losses in the transmission system and the revs will slow down - the "spare" energy will be the heat caused by friction between your fingers and the spindle. Now suddenly remove that loss and the motor suddenly has all of it's energy back and spins up quickly again. Probably not the best comparison in the world, but hopefully you get where I'm coming from. Quote Link to comment Share on other sites More sharing options...
Lurksalot Posted July 20, 2005 Share Posted July 20, 2005 so whats the difference between a snapped gearbox shaft and quickly depressing the clutch at revs Quote Link to comment Share on other sites More sharing options...
Nick M Posted July 20, 2005 Share Posted July 20, 2005 The speed at which it happens, throw of the clutch pedal, etc. - there'll be a small (tiny even) element of slip as the clutch is disengaged and that's maybe all the time the ECU needs to compensate and for the rev-limiter have the desired effect. To be honest I suspect that dipping the clutch at 8,000rpm would still give the ECU a hard time. Try it and let me know what happens... Quote Link to comment Share on other sites More sharing options...
Lurksalot Posted July 20, 2005 Share Posted July 20, 2005 Let me get a car first it would be no problem on the bike red line 14k Quote Link to comment Share on other sites More sharing options...
Mike-SEiW Posted July 21, 2005 Share Posted July 21, 2005 This is an interesting discussion If an engine is spinning at X rpm with load, load is removed and revs accelerate, if fuel and spark is removed, revs will NOT accelerate any more from that point. Acceleration of revs will stop the instant fuel and/or spark is removed. You cannot accelerate anything without putting in energy. Acceleration does not have "inertia" in the same way that speed (i.e. something containing kinetic energy) does. This is pure physics and very simple (F=ma). So if rev limiter is at 9k, the engine with or without load cannot rev more than that unless it's being forced (by being in a lower gear for instance). Quote Link to comment Share on other sites More sharing options...
Nick M Posted July 21, 2005 Share Posted July 21, 2005 You cannot accelerate anything without putting in energy. Acceleration does not have "inertia" in the same way that speed (i.e. something containing kinetic energy) does. Yep - agree with all of that. I didn't mean that the acceleration has inertia - I realise that the bottom end won't continue to spin faster and faster if you don't keep supplying energy to it - but once you're in an over-rev situation the inertia of the bottom end will mean the ECU cannot slow the engine down again - it cannot extract energy from the system. If the car is accelerating hard at WOT and suddenly the load is removed then the lag in the time it takes for the ECU to *stop* supplying fuel and spark might be enough for the engine to buzz past the rev-limit. Maybe not for long, and maybe not by much, but it certainly could happen. Quote Link to comment Share on other sites More sharing options...
oioi Posted July 21, 2005 Share Posted July 21, 2005 another way to look at this is to work out the potential over revs (and I agree with Nick the engine will over rev) hypothetical engine - full throttle accelerating hard in second gear at 7000rpm input shaft snaps. redline is 7500. Time taken to go from 7000 to 7500 rpm is x (remeber now no load on engine). (for the sake of argument no soft limit). Time taken for the processor to see 7500, decide to cut fuel and cut the fuel is y. during the time y the engine will continue to increase revs above 7500. so if we know response time of the system (y) and the rate of increase of revs at full throttle (500/x) at 7500 we can calculate the over rev of the engine. hopefully this makes sense to someone other than me. now if we had some real data of system response time (someone fancy pm'ing bill shurvinton?) we could actually calculate the over revs OiOi Quote Link to comment Share on other sites More sharing options...
Nick M Posted July 21, 2005 Share Posted July 21, 2005 Chances are the ECU would cut the spark rather than fuel, but it might do both. Haven't really bothered to find out how mine works, other than checking it does. Quote Link to comment Share on other sites More sharing options...
oioi Posted July 21, 2005 Share Posted July 21, 2005 okay - but we still need to know the response time to cutting spark Quote Link to comment Share on other sites More sharing options...
Nick M Posted July 21, 2005 Share Posted July 21, 2005 okay - but we still need to know the response time to cutting spark Indeed I was being pedantic Quote Link to comment Share on other sites More sharing options...
scruffythefirst Posted July 21, 2005 Share Posted July 21, 2005 another way to look at this is to work out the potential over revs (and I agree with Nick the engine will over rev) hypothetical engine - full throttle accelerating hard in second gear at 7000rpm input shaft snaps. redline is 7500. Time taken to go from 7000 to 7500 rpm is x (remeber now no load on engine). (for the sake of argument no soft limit). Time taken for the processor to see 7500, decide to cut fuel and cut the fuel is y. during the time y the engine will continue to increase revs above 7500. so if we know response time of the system (y) and the rate of increase of revs at full throttle (500/x) at 7500 we can calculate the over rev of the engine. hopefully this makes sense to someone other than me. now if we had some real data of system response time (someone fancy pm'ing bill shurvinton?) we could actually calculate the over revs OiOi Seems like the rev limiter should be set at the maximum revs the engine can survive without damage, minus the response time with no load, and not just set at the max rpm the engine can take. Quote Link to comment Share on other sites More sharing options...
Toby Mack Posted July 21, 2005 Share Posted July 21, 2005 not sure I buy Nick's arguement. The ECU has to respond fast enough to time the spark. To do that it has to be able to accurately detect events of a degree or so at any revs. The only way this could be the case is if the ECU deliberately waits a few cycles before cutting spark/fuel. Looking at the code for megasquirt at least I don't think it does that. it measures revs each cycle and checks for the rev limit continuously in the main program loop. Toby Quote Link to comment Share on other sites More sharing options...
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