Engine Braking

  1. I am starting to program a simple physics model of a car to help me understand how it works. I am certainly going to have to make quite a few simplifications as I'm only just about to start A levels.

    Whilst researching how to simulate engine braking I came across the Racer (racing simulator) physics reference page which says...

    Is this an accurate statement, and if so what are those compression effects? Why do they result in Engine Braking [tex]\propto[/tex] Engine Speed?

    Finally, the site suggests that braking torque = braking coefficient x rps, is this accurate and how can a braking coefficient be calculated or approximated?

    Thanks,
    Leo
     
  2. jcsd
  3. mgb_phys

    mgb_phys 8,952
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    You know how a piston engine works?
    Inject fuel, fuel burns, expands, pushes down piston. Piston comes back up and pushes exhaust gases out.
    Now imagine it with no bang.
    Air comes into the piston, the piston goes down trying to pull against the partial vacuum created, the piston then goes up trying to compress the gas in the cylinder.
    (This might be oversimplified for modern fuel injection engine with electronically controlled valves)
    The amount of torque needed to push the pistons around will depend on the capacity of the engine and the gear ratio but would be proportional to the engine RPM because that is simply the number of piston compressions per second.
     
  4. Right OK, that's pretty obvious now I come to think of it! Thanks.
     
  5. rcgldr

    rcgldr 7,414
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    Most of the engine braking effect is due to the movement of air underneath the pistons, not above them. You can try this for yourself. Coast down a hill, turn off your engine, and vary the throttle settings, there will be no perceptible difference in engine braking. The air is reasonably elastic, so the losses above the piston are small compared to losses below the piston, where air is being moved back and forth between the pistons in an engine.

    Pro-street drag motorcycles use a vacuum pump to evacuate the crankcase to reduce the loss of power from the movement of air underneath pistons.

    This article on Jake Brakes explains it: "forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft"

    http://en.wikipedia.org/wiki/Jake_brake

    Devices like Jake Brakes release compression at the right time to enhance engine braking, but few cars have this. On two stroke engines, just a compression release will increase engine braking (although the purpose is to make it easier to start the engine).
     
    Last edited: Aug 7, 2007
  6. mgb_phys

    mgb_phys 8,952
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    Interesting - the friction of air flowing in the crankcase is greater than the compression of the air in the cylinders?

    Before trying this make sure you don't have a steering lock and you know how little braking/steering you have left without power!!!!
     
  7. AlephZero

    AlephZero 7,300
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    I would have thought the largest engine braking effect was the friction between the piston rings and cylinders. Consider how rapidly an idling engine stops when the ignition is switched off. It's hard to believe that deceleration comes mainly from pushing a bit of air around under the pistons, at low RPM. Turning an engine over by hand (with the ignition OFF!!!) will show you how much torque is needed to overcome friction. Removing the spark plugs so there is no compression doesn't make much difference.

    Of course if you want to screw the last 0.1HP out of your engine, reducing the crankcase pressure may help - I admit I know nothing about drag racing engines.
     
  8. Danger

    Danger 9,879
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    We used pan-evacuation systems on cars, too. It's worth a couple of percent increase in hp (not sure how much, but it's probably comparable to an H-pipe in the exhaust). Every little bit helps.
     
  9. rcgldr

    rcgldr 7,414
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    Pro stock bikes rev around 13,000 rpm, it's enough that Vance and Hines used 3 vacuum pumps on their bike until the rules limited the number of vacuum pumps to just one.

    Yes, there's a lot of friction. I was just pointing out that the movement of air back and forth below the pistons has much more braking effect than compressing air and letting it push back on the pistons.
     
  10. BobG

    BobG 2,340
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    Yes, definitely!!!

    Periodically, we have tourists use their brakes exclusively to descend Pikes Peak (instead of using a lower gear that would provide the engine braking you're talking about). The sneaky ones manage to bypass the mandatory brake checkpoint a little over halfway down (you have to have the temperature of your brakes checked by a forest service technician).

    The reaction of a driver when their brake fluid boils on the way down a mountain can be poignantly pathetic, especially when the car is filled with the wife and kids of the driver. It's almost a haunting effect to be driving up the mountain and have a car come rolling silently down the mountain and to stare at the terror filled faces peering desperately out the window. Obviously, a driver couldn't possibly slam their car into park as it rolled down the hill at 45 - 50 mph. That would ruin their transmission. Better to shut off the ignition, lock up the steering wheel, and hope the car can withstand plunging off the side of a cliff once the first turn arrives.
     
  11. mgb_phys

    mgb_phys 8,952
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    Used to work on an observatory that was about 2500m above and an hours drive from the city - you could do the run entirely in freewheel with practice.
    One day somebody decided that they might as well turn the engine off since they weren't using it, first hair pin bend the steering lock went on -fortunately they went into the side of the mountian, not over the edge.
     
  12. Can you explain to me how you can get the steering to lock without putting the vehicle in park? Any vehicle I've ever been in with an automatic transmission and a steering lock you CANNOT lock the steering without putting the vehicle in park just for the reasons discussed above.
     
  13. mgb_phys

    mgb_phys 8,952
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    Who drives an automatic when your commute to work involves going up a 2500m mountain? Most manual cars go into parking lock as soon as you tunr the key past off and turn the wheel more than about 1/8 turn.
     
  14. Danger

    Danger 9,879
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    In any event, just the loss of the steering boost can be quite unnerving on a hill, even if it isn't actually locked.
     

  15. I've never seen a stick-shift with a steering lock that doesn't have the little extra button on the opposite side of the steering column from the key which needs to be pushed in order to get the key to all the way back to the lock position.
     
  16. mgb_phys

    mgb_phys 8,952
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    You must be in the USA - I remember having a hire car there once with so many safety interlocks ie. must be in park and the hand brake on and press the foot brake all the way to the floor while having both hands on the steering wheel to turn the engine on.

    It is amazing how much steering and brake force you lose with the engine off as Danger said, I just moved my wife's new car a couple of m down the drive by putting it in neutral and letting it roll - my first experience of expensive car with power everything.
     
    Last edited: Aug 8, 2007
  17. rcgldr

    rcgldr 7,414
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    This is getting off topic. I've actually done this on a car with a stick shift. Since this is a thread about engine braking, I'm ignoring cars with limited engine braking, such as slipper clutches used on some race cars. If you're getting significant engine braking, then the engine rpms are reasonably high enough for power steering and power brakes to work just fine.

    Regarding the steering lock, this depends on the car. Newer cars with keyless systems and just buttons for starting the engine don't have steering lock at all (for obvious safety reasons). Then again, I'm not sure you can turn off the engine on an all button car while the car is moving. For cars with steering locks, check to see if the engine can be turned off without engaging the steering lock while the car isn't moving.

    So my suggested experiement is limited to those who own cars that don't have a steering lock issue. As an alternative a motorcycle could be used to try this experiement, or you can just take the word of the guys that did the wiki article about Jake Brakes that I linked to earlier.

    Also if you try this experiment on a carbureted engine, you could fill the exhaust with gas fumes that could blow holes in the exhaust when you turn the engine back on. Stop quickly and restart the engine after waiting a bit.
     
    Last edited: Aug 9, 2007
  18. mgb_phys

    mgb_phys 8,952
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    At the risk of getting on topic - is my understanding of modern engines correct?
    In neutral with no load the engine runs normally at <1000rpm burning fuel.
    Going downhill in gear but with no accelarator the engine turns at the same RPM that it would normally run at, in that gear at that speed.
    No fuel enter the cylinders but the valves open and close on each cycle so the air is only compressed once then re-expands - since this is fairly elastic the main losses are friction in the engine and gearbox/transmission.
    But in an old carburetor system the same amount of fuel would enter as on idle (the slow running jet) and the the spark plugs would still fire, so presumably this fuel would burn in a very dilute mixture, since the RPM is much higher.
    What happens on a modern fuel injection car? Does it put any fuel in at all when in gear with no accelerator?
     
  19. rcgldr

    rcgldr 7,414
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    Plus all the movement of air back and forth between cylinders underneath the pistons. At higher rpms, most of the engine braking is due to this movement of air.
    Depends on the car. My wife owned one such car, a 1989 Ford Probe that completely shut off the gas during engine braking with no throttle input, the instant fuel milage readout would suddenly transition to 99mpg, it's max reading, when the fuel was shut off.

    Off topic, one other feature that the Probe had was a automatic tranny with an internal mechanical clutch to bypass the fluid clutch one the car was "cruising", it felt similar to having an extra gear in the car and improved gas milage (fluid clutches consume energy). I don't recall if the mechanical clutch remain engaged during engine braking. I also don't remember this lock up clutch feature an any of the cars she's since bought after the Probe. All of the cars I've owned since 1980 are stick shift.
     
  20. AlephZero

    AlephZero 7,300
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    Mine does some of the time, at least. (UK, stick shift, small Vauxhall/GM built in 2000).

    With no accelerator it tries to maintain the normal engine idle speed (about 800 RPM). That's quite useful when creeping along in traffic in 1st/2nd/3rd gear. Almost like low-speed cruise control.

    I don't have a trip computer, but from the way the car behaves when decelerating slowly with no accelerator, I suspect it starts to feeding in fuel when the RPM drops to about 1500. There seems to be a step change in the rate of decel at about 1500RPM (though it's not noticeable with moderate or hard braking).
     
  21. BobG

    BobG 2,340
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    Actually, the engine RPM depends on the car and the gear. Going downhill and taking your foot off the accelerator, the car will noticeably slow (if the gearing is low enough), but it eventually starts to speed back up, although at a lot slower pace than if you were in neutral. The engine might try to maintain an idle speed, but it only resists acceleration due to gravity, not negates it. (It's probably more accurate to say the perception of the RPM depends on the gear. Low gears definitely resist acceleration a lot better than high gears.)

    And even fuel injection cars inject enough fuel to at least maintain idle speed. In fact, mine will go uphill with no accelerator when in 1st gear and 4WD Low.
     
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