29 April 2015

On engine braking

I'm not sure what engine braking is.

I know what it does and I can apply the effect, but I'm not sure what causes it, the physics behind it, you know.
And I want to know because I want to understand why 2T has got so little of it and why it's configurable on 4T engines.

Yes, engine braking is a configurable setting on some/most/all (?) ECU equipped bikes.
I have never seen any manufacturer offer ECU programming software on off-the-shelf bikes, so I guess most people have never seen it's configurable; but at least YEC and HRC offer it on their race-spec ECU's and I can't imagine the others wouldn't.

Anyways, let's see what the internet can tell me about engine braking.

From Wikipedia:
"The term 'engine braking' refers to the braking effect caused by the closed-throttle partial-vacuum in petrol (gasoline) engines when the accelerator pedal is released. While some of the braking force is due to friction in the drive train, this is negligible compared to the effect from the vacuum.

When the throttle is closed, the air flow to the intake manifold is greatly restricted. The concept can be illustrated by the amount of effort required to blow/suck through a thin tube vs. a wider one. It is the work the engine has to do against this restricted air flow that provides the braking effect."

"As soon as the accelerator is released and the throttle closes, engine braking comes into effect as long as the wheels remain connected via the transmission to the engine."

"The braking force varies depending on the engine, but also what gear the vehicle is in."

"Additionally, fuel injected engines don't use any fuel while engine braking. This is known as DFCO or Deceleration Fuel Cut-Off."

Doesn't explicitly state what causes it, but at least I've extracted some key points:
  • no fuel usage during engine braking
  • lack of air flow makes the 'engine' do something to invoke engine braking
  • engine braking only occurs when the drive train is connected to the engine and the tire connected to the road
  • transmission plays a part (the lower the gear -> the higher the engine braking)

I'm going to assume it's the exact opposite of accelerating (I've got to start somewhere figuring this out), which makes the slowing down effect must come from the 'engine' as the Wikipedia so eloquently states. But what is the 'engine' in this case?

Acceleration comes from the down movement of the piston in the cylinder due to the rapidly expanding gas mixture combustion inside a vacuum chamber.
  • The vacuum chamber comes from all the valves being closed as being one phase in a four-stroke's cycle.
  • The mixture comes from air/fuel being mixed and injected.
  • The combustion comes from from the spark plug igniting the gas/air mixture
But we're not accelerating, we're engine braking.

Maybe that's it: the fact that during that exact stroke, everything is absent that otherwise makes it accelerate:
  • we've closed the throttle, there is no gas/air mixture in the combustion chamber.
  • there is no spark to ignite anything
  • there is no compression, there is just vacuum
That's it! Expanding a vacuum is really hard, right?
The drive train still is connected throughout the entire time.
Inertia makes the back wheel keeps moving so everything connected to the wheel is moving, still.
The chain is moving, the transmission is moving and ultimately, the piston itself it going up and down still.
And that must be a pretty hard thing to do, the wheel pulling the piston down trying to expand the vacuum inside the combustion chamber and there's the deceleration effect right there.

Now I get it!
It's configurable because the injection system can still inject a certain amount of air inside the chamber right before the 'pull' to make it easier to expand the other wise semi-vacuum chamber.
The more air injected before the 'pull' stroke, the easier it'll be to pull down on the whole, the less pronounced the engine brake effect will be.

And now I get why 2-strokes haven't got as much engine braking, to the point where it's claimed there's none at all.
It's because in a 4T system, there's a completely separate phase where the entire system is closed down, creating a lot of time for the back wheel to try and pull the piston down, expanding the semi-vacuum and that never really happens in a 2T system. 
It kind of does, but it doesn't nearly last as long as in a 4T system, making the engine brake effect negligible.