Engines break when you don't take care of them...
Assuming that the seamless shift is similar to the zero shift
http://www.f1network.net/main/s491/st129248.htm
The "bullets" tend to engage the higher gear during engine braking, so there's still some form of limited engine braking.
The above explanation of the zeroshift transmission, and other articles about seamless shift indicate tht the clutch is slippped during each shift.
No, more than just idle control. I can accelerate up to 25mph to 30mph, and lift off the throttle, and the computer controlled slowed ramp down of virtual throttle position prevents the car from slowing significantly, fighting the brakes. I usually have to use the clutch to slow down the car in these situations.
Traction control in a street situation, with cold tires is different than traction control in a race situation.
Otherwise, they'd be prone to oversteer, and oversteer while pulling close to 4g's at 160 mph would be considered hazardous to most, since once the car yaws (oversteers) enough, the downforce is greatly reduced because air flows over the wings at the wrong angle. For this reason, any "sane" (safe) setup for a high powered, high downforce car is going to have a rearwards aero bias, to reduce the chance of oversteer in high speed, high g force turns.
ok, so basicaly, you are all saying exactly what I said. but adding a whole load of other information to it.
As far as the one post goes about the engine slowing down the same regardless of throttle positon when the ignition is off, I'd like to say thats probably a rare case since EVERY vehicle I have ever driven has only had full engine braking with the throttle closed.
I rode my motor bike down the road recently and switched the engine off while the throttle was open, it didn't start to engine brake untill I closed it, so I doubt it's any different on any other four cycle engine. I bet the engine braking is less on most cars even, since the bike litteraly throws you onto the seat if you arn't careful.
As far as I know diesel cars run some form of engine brake, because alot of people who drive manuals use engine braking, they have to have some form on diesel cars. This is because if someone is expecting it and it doesn't happen it would cause an accident. It'd be pretty bad on hills too, since a diesel will accelerate on a down hill even with the throttle off unless they have an EB equiped.
I know the newer larger ford F-series have them, they use the VGT to restrict exhaust. so they essentialy have an exhaust brake.
Get in any larger diesel vehicle and I garantee they will either have a switch on the dash labeled "Jake Brake" or "Engine Brake", or they won't have alot of engine braking at all when you let off.
The air in the crank case does cause a small amount of resistance, but far smaller than you'd think, gaining 30+hp from a vacuumed crank is simply because you are reducing the total air friction on the moving parts, but those parts usualy have enough mass and thus inertia to overcome the air risistance comming off the throttle.
One more thing I forgot to mention, on a vacuumed crank engine braking will naturaly be reduced because the air pressure under the pistons will be closer to the air pressure above, this means the actual resisting force will be reduced. That could lead someone to think it's the air movement in the crank that causes braking when it isn't, it's the vacuum above.
And as far as F1 not allowing engine braking reduction, well good. I personaly think the more crap they add the less the sport.
F1s experiance alot of engine braking for exactly the reason Bob mentioned, massive bore, short stroke. There is a huge surface area on both the top and bottom of the pistons, so the air has alot of surface area to act on.
I would love to see a modern F1 driver drive a vintage 60's F1, they'd have a hell of a time I'm sure. People seem to think just because the cars got faster it automaticaly means they got harder to drive. I don't realy think thats the case.
I'll admit, it takes a damn good driver to handel an F1, only a small number of people can, but the modern ones do make that number slightly larger.
As far as the one post goes about the engine slowing down the same regardless of throttle positon when the ignition is off, I'd like to say thats probably a rare case since EVERY vehicle I have ever driven has only had full engine braking with the throttle closed.
I rode my motor bike down the road recently and switched the engine off while the throttle was open, it didn't start to engine brake untill I closed it, so I doubt it's any different on any other four cycle engine. I bet the engine braking is less on most cars even, since the bike litteraly throws you onto the seat if you arn't careful.
As far as I know diesel cars run some form of engine brake, because alot of people who drive manuals use engine braking, they have to have some form on diesel cars. This is because if someone is expecting it and it doesn't happen it would cause an accident. It'd be pretty bad on hills too, since a diesel will accelerate on a down hill even with the throttle off unless they have an EB equiped.
I know the newer larger ford F-series have them, they use the VGT to restrict exhaust. so they essentialy have an exhaust brake.
Get in any larger diesel vehicle and I garantee they will either have a switch on the dash labeled "Jake Brake" or "Engine Brake", or they won't have alot of engine braking at all when you let off.
The air in the crank case does cause a small amount of resistance, but far smaller than you'd think, gaining 30+hp from a vacuumed crank is simply because you are reducing the total air friction on the moving parts, but those parts usualy have enough mass and thus inertia to overcome the air risistance comming off the throttle.
One more thing I forgot to mention, on a vacuumed crank engine braking will naturaly be reduced because the air pressure under the pistons will be closer to the air pressure above, this means the actual resisting force will be reduced. That could lead someone to think it's the air movement in the crank that causes braking when it isn't, it's the vacuum above.
And as far as F1 not allowing engine braking reduction, well good. I personaly think the more crap they add the less the sport.
F1s experiance alot of engine braking for exactly the reason Bob mentioned, massive bore, short stroke. There is a huge surface area on both the top and bottom of the pistons, so the air has alot of surface area to act on.
I would love to see a modern F1 driver drive a vintage 60's F1, they'd have a hell of a time I'm sure. People seem to think just because the cars got faster it automaticaly means they got harder to drive. I don't realy think thats the case.
I'll admit, it takes a damn good driver to handel an F1, only a small number of people can, but the modern ones do make that number slightly larger.
when porsche launched the 968 they said that they had gained 5hp by optimising the airflow through the crankcase so that the air being pushed down by the decending piston was vented to the area under the piston that was rising at the same time so gaining by reducing the power needed to push the air down and by reducing the low preassure area forming under the rising piston
its abolutely impossible for the rear wheels to lock up from engine braking since the engine would have to stall for that to happen
I agree with DragonCommando that the engine braking in petrol engine is due to vacuum created by closed butterfly valve, but still i have a doubt...I was coasting downhill with high rpm and felt high engine braking as soon as released my foot from the throttle, then i switched off the ignition and pressed the throttle wide open hoping that the engine braking will vanish because now there is no restriction to the intake valve and thus no vacuum is created....but to my surprise there was no difference in engine braking with closed or open throttle (with ignition off)....and my car accelerator does not have throttle by wire, it has direct mechanical linkage by wire to the butterfly, so i am confident that on pressing the accelerator the valve did open but engine braking was same as with closed throttle. I tried the same experiment many times and got the same result.
It will be appreciated if DragonCommando can explain why this is happening because the engine braking should reduce on opening the throttle with ignition off to my understanding of the concept.
It will be appreciated if DragonCommando can explain why this is happening because the engine braking should reduce on opening the throttle with ignition off to my understanding of the concept.
(Forbin)
DELETED
by Forbin
OK, here's a related question.
I'm told that most modern engines shut off the fuel completely when you lift off the throttle; I asked this question a few years ago on here. Two implications of this are:
I'm told that most modern engines shut off the fuel completely when you lift off the throttle; I asked this question a few years ago on here. Two implications of this are:
- that when you coast down a hill in gear, you're using no fuel
- it's better to coast up to junctions in gear than to drop it into neutral and roll up with the engine idling (as it needs a small amount of fuel to keep it idling).
I'm no expert but afaik the only location you really hear the combustion is out of the exhaust (and even there it is usually extremely muffled)...
Indeed. Most engine noise is 'induction' noise, the mechanical whirring of all the components, road noise and wind noise. When I lift off, my engine does sound different.
The ECU is reducing the time fuel injector is open, its less than at idle, but its still not closed completly. Atleast on Toyota's engine it is working like this
Pretty sure it has to turn off injection completely, as reducing the injection time and amount of fuel would cause the engine to run lean (and possibly destroy itself). All or nothing here.
Depends on the car, but from tests I did I found some fuel still used. My 330d did around 600mpg downhill off throttle.
i did a back to back with my current car on the same slope. In gear it did 98mpg, coasting was 85mpg.
Coasting has an advantage of not having engine braking, so you can go faster and further without pressing the throttle. Whether that translates to lower overall mog would depend on the slope, speed and whatnot.
i did a back to back with my current car on the same slope. In gear it did 98mpg, coasting was 85mpg.
Coasting has an advantage of not having engine braking, so you can go faster and further without pressing the throttle. Whether that translates to lower overall mog would depend on the slope, speed and whatnot.
I doubt many engines would actually have any problems at all from running lean on the overrun. Happens all the time on carbs (the popping noise). It's not like EGTs are high, and knock limits won't be an issue.
Engine wont destroy itself by lean mixture during engine braking. Lean mixture is only dangerous when there is more load.
Fuel injection systems have been cutting 100% of the fuel during engine braking from the start of their era. It was one of the major selling points versus the carbs, for every day customers because it allows "big" fuel savings.
Your engine will still make noise because it is running as a kind of an air pump. It still passes air thru it and they pulse to the exhaust with the rhythm created by the exhaust valves opening. There just isn't any combustion. And plus the mechanical noises.
Checking the mpg readings on a car's driving computer isn't an accurate way of determining how much the injectors are open at the moment.
Fuel injection systems have been cutting 100% of the fuel during engine braking from the start of their era. It was one of the major selling points versus the carbs, for every day customers because it allows "big" fuel savings.
Your engine will still make noise because it is running as a kind of an air pump. It still passes air thru it and they pulse to the exhaust with the rhythm created by the exhaust valves opening. There just isn't any combustion. And plus the mechanical noises.
Checking the mpg readings on a car's driving computer isn't an accurate way of determining how much the injectors are open at the moment.
Exactly, my information is based on real-time stats of the engine with laptop plugged in. Its 50msec while idle, and 25 msec when off throttle. I wasn't doing it myself, just a guy on a Corolla forum did these researches.
50 ms at idle? No way, I just checked one of my own Megasquirt logs and my car runs under 2 ms opening time at idle (reasonably big injectors tho) and even at 1.6 bar boost @ 6000 rpm the injector opening time is 7.6 ms with 80% duty cycle.
-EDIT- Are you sure the values weren't 5 ms and 2.5 ms? This way the values could make sense if the injectors are a lot smaller than mine. Can't really explain where the 2.5 ms at overrun comes from tho. Makes no sense really. Only thing what comes to mind is that Toyota wants to cool down the engine with squirting a little petrol in there but not igniting it. I just would assume this not being really that necessary.
-EDIT- Are you sure the values weren't 5 ms and 2.5 ms? This way the values could make sense if the injectors are a lot smaller than mine. Can't really explain where the 2.5 ms at overrun comes from tho. Makes no sense really. Only thing what comes to mind is that Toyota wants to cool down the engine with squirting a little petrol in there but not igniting it. I just would assume this not being really that necessary.
Probably, I don't remember exactly.
About the fuel consumption when in gear and when idle. My fathers Citroen C4 still shows some fuel consumption when driving downhill in a gear. I think it's around 1L/100kn (which seems pretty much actually) at ~60km/h.
Also, another question from me. Why does the fuel consumption increase when you put it in neutral then press clutch? It's around 0.5L/100km higher then when it is only in neutral without pressing the clutch.
Also, another question from me. Why does the fuel consumption increase when you put it in neutral then press clutch? It's around 0.5L/100km higher then when it is only in neutral without pressing the clutch.
Could be some sort of ECU strategy for better clutch engagement. If you're in neutral with the clutch up, the ECU assumes you just want a steady idle. But if you press the clutch, it thinks "you're going to engage a gear and start moving soon, so I'll prepare by advancing the ignition and injecting a tiny bit more fuel". Perhaps. ECUs can do lots of clever things these days to make cars more efficient, more user-friendly and less prone to stalling, vibration etc.
It's kind of up to the manufacturer as to if the fuel is cut completely or not. Automatic vs Manual transmission also plays a part in it.
I can tell you my Porsche 944 cuts fuel completely until the engine speed drops to 1300rpm then fires again and the engine drops softly to idle. I can program that to any RPM I want. My diesel truck is kind of the same way. It closes off the fuel inlet passage to the injection pump but it's not a perfect shutoff and a very small quantity of fuel sneaks through on those high rpm downshifts. Fuel resumes when the rpm drops to 1000. In either case, you can turn the ignition off and it makes no difference in either.
There really isn't one single thing that causes engine braking by itself. There are many little things that cause the engine to reduce speed. It's trying like hell to suck in air, then it has to squeeze that (most energy is returned from the spring effect but there is still loss), then it has to force all that air out another even smaller hole than it sucked it in from.
Then there is friction and quite a lot of it. Think of all the moving parts that are rubbing together. Don't forget you are pumping oil and usually water around which takes energy.
Air displacement when the pistons come down still takes energy no matter what. Think of air as the fluid it is, it takes some force to move out of the way even if another is piston is assisting in sucking from the other side. We don't live in a perfect universe, sorry. Entropy is kind of annoying. There's more but I cannot be bothered to keep typing. Just think about it and remember we do not live in a perfect world.
I can't be bothered to list everything. Different engines act differently as well but you still have the basics. If you can't wrap your head around a 4-stroke or diesel then start with a 2-stroke and move your way up. They don't have as much engine braking but it's still there.
I can tell you my Porsche 944 cuts fuel completely until the engine speed drops to 1300rpm then fires again and the engine drops softly to idle. I can program that to any RPM I want. My diesel truck is kind of the same way. It closes off the fuel inlet passage to the injection pump but it's not a perfect shutoff and a very small quantity of fuel sneaks through on those high rpm downshifts. Fuel resumes when the rpm drops to 1000. In either case, you can turn the ignition off and it makes no difference in either.
There really isn't one single thing that causes engine braking by itself. There are many little things that cause the engine to reduce speed. It's trying like hell to suck in air, then it has to squeeze that (most energy is returned from the spring effect but there is still loss), then it has to force all that air out another even smaller hole than it sucked it in from.
Then there is friction and quite a lot of it. Think of all the moving parts that are rubbing together. Don't forget you are pumping oil and usually water around which takes energy.
Air displacement when the pistons come down still takes energy no matter what. Think of air as the fluid it is, it takes some force to move out of the way even if another is piston is assisting in sucking from the other side. We don't live in a perfect universe, sorry. Entropy is kind of annoying. There's more but I cannot be bothered to keep typing. Just think about it and remember we do not live in a perfect world.
I can't be bothered to list everything. Different engines act differently as well but you still have the basics. If you can't wrap your head around a 4-stroke or diesel then start with a 2-stroke and move your way up. They don't have as much engine braking but it's still there.
I think it is easier to learn how a four stroke engine works first...four strokes of the piston- suck, squeeze, bang, blow. Two strokes can be far more confusing.
Engine braking: How it really works
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