Engine mechanisms during coasting (in drive mode) and engine braking

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  • Thread starter iVenky
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  • #1
iVenky
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I am trying to understand what happens in the engine (especially gasoline vehicles) during coasting (in drive mode) and engine braking. It looks like both mechanisms shut off the fuel injectors. Then how is the former (coasting in drive mode) able to use the momentum while the latter (engine braking) could apply a brake?
 

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  • #2
Halc
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I can be engine braking (moving in top gear, foot off the gas) and the car indeed slows, but if I shut off the ignition while doing this, the car brakes more. This implies that my fuel injectors are not actually shut off in the former case.

Coasting (as opposed to engine braking) usually involves disengaging the engine (via clutch or neutral gear).
 
  • #3
AZFIREBALL
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If you take your foot off the accelerator while driving down a level highway in top gear, I think the throttle is positioned at the engine’s idle point. And the fuel being supplied to the engine is that required for the engine to idle. (This may be different on computer controlled engines.)

If the engine is not disconnected from the drive train, you will feel deceleration as a result of the engine's drag on the drive train, since it is still being rotating at a relatively high RPM by the forward momentum of the car. If you shift to a lower gear at this point, the deceleration will increase, due to the car’s drive train now causing the engine to rotate faster. This deceleration increase will occur as you shift to each lower gear, as the engine provides more drag due to increased RPM.

This is generally called ‘engine braking’. There is also deceleration due to wind resistance, drive train frictions and rolling resistance.

If you disengage the engine from the drive train, while removing your foot from the accelerator; this is generally called ‘coasting’. There will still be some deceleration due to wind resistance, drive train frictions and rolling resistance; but no engine braking.
 
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  • #4
russ_watters
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In the cars I've had where I've been able to access the telemetry, when i take my foot off the gas, fuel flow drops to zero. My current car has a second mode (which i shut off) that disengaged the engine for coasting and a third that puts you in a lower gear for more resistance.

What kind of car do you have that you can shut off the ignition while moving?
 
  • #5
AZFIREBALL
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I do not think it advisable to turn off the ignition if the injectors or carburetor is still providing fuel to the engine. The reason is unburnt fuel is being pumped into the tail pipe and catalytic converter (on some cars). When the ignition is turned back on, it might blow a hole in your exhaust. We used to do this on our motorcycles to get a loud bang from the tail pipe to scare the cows along the road.

russ_watters: What make car has the functions you noted?
 
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  • #6
russ_watters
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AZFIREBALL said:
russ_watters: What make car has the functions you noted?
The different coast modes? I'm driving a 2019 Kia Stinger, but I think these features are common on new cars today.

It also shuts off the engine when I stop.
 
  • #7
tech99
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I think it is a good idea for the drive train (and the driver) to keep the wheels connected to the engine when "coasting". In this way, the momentum of the rotating engine and flywheel wil make it less likely the road wheels cold stop turning or lock. In such a case, steering is impaired or lost.
 
  • #8
iVenky
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If you take your foot off the accelerator while driving down a level highway in top gear, I think the throttle is positioned at the engine’s idle point. And the fuel being supplied to the engine is that required for the engine to idle. (This may be different on computer controlled engines.)

If the engine is not disconnected from the drive train, you will feel deceleration as a result of the engine's drag on the drive train, since it is still being rotating at a relatively high RPM by the forward momentum of the car. If you shift to a lower gear at this point, the deceleration will increase, due to the car’s drive train now causing the engine to rotate faster. This deceleration increase will occur as you shift to each lower gear, as the engine provides more drag due to increased RPM.

This is generally called ‘engine braking’. There is also deceleration due to wind resistance, drive train frictions and rolling resistance.

If you disengage the engine from the drive train, while removing your foot from the accelerator; this is generally called ‘coasting’. There will still be some deceleration due to wind resistance, drive train frictions and rolling resistance; but no engine braking.

Hi,

Thanks for the reply. Sorry, I wasn't clear with my question before. When I meant coasting (i didn't mean the neutral gear when the transmission is disconnected), I was talking about the case you just release the foot of the accelerator. How is this case different from engine braking?
 
  • #9
Halc
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I was talking about the case you just release the foot of the accelerator. How is this case different from engine braking?
It is the same as mild engine braking. Usually the latter also involves down-shifting, which increases the resistance.
 
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  • #10
Whipley Snidelash
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In the cars I've had where I've been able to access the telemetry, when i take my foot off the gas, fuel flow drops to zero. My current car has a second mode (which i shut off) that disengaged the engine for coasting and a third that puts you in a lower gear for more resistance.

What kind of car do you have that you can shut off the ignition while moving?

you can shut the engine off while you’re driving in my manual shift C6 Corvette, The steering column doesn’t lock and there’s no key, you will have to fight the steering a bit because then the power steering is off but everything works
 
  • #11
Averagesupernova
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you can shut the engine off while you’re driving in my manual shift C6 Corvette, The steering column doesn’t lock and there’s no key, you will have to fight the steering a bit because then the power steering is off but everything works
If the engine is still turning you will have power steering.
 

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