What brakes the ICE? Calling Patrick, Bob & other experts

Hi Guys,

This question really belongs in an overall Prius Technical Discussion group, but there doesn't seem to be one.

Following my recent Brake Warning lights and ECU problem, I have been reading a number of the threads relating to braking issues, and the problems with the transition from Regen to Friction braking, and Regen/ABS/VSC interactions and so on. I have to say, from what is described, there seems to me a design flaw, as it all seems quite illogical, but I'll go into that another time and place.

For now, my question, which I haven't seen discussed anywhere, is this:

When the ICE is turned off, at low vehicle speeds, the crankshaft obviously (okay, apparently, would make sense logically) stops turning.

However, it's still connected to one end of the Planetary gear set, so is subject to (potentially high?) reactive torques, particularly when the vehicle is enjoying forward regen braking.

In that situation, the torque from the wheels is opposite to the normal drive torque (overrun), so will be acting to drive the ICE forwards, instead of the ICE driving the wheels forward. This would cause the ICE to turn slowly, or even quickly, (thereby wasting precious energy) if it weren't being held stationary by some form of brake.

That's my question, as I have never seen any discussion on this brake. But I presume there must be one? What form does it take, where is it located, and how is it operated, please?

Also, not directly related, the friction brake mechanical booster - is this pressure rather than vacuum driven, and where is the pump? And does anyone know the operating pressure?

Regards,
Al

 

Howdy Easy Rider!

Yes, but... Under applied torque such as will be generated by high regen braking, surely the engine will turn? It's designed for low friction, and the oil is thin...

Indeed, it continues to turn (continually) whether fuel is being injected or not, at highway speeds (over 60-odd km/h, I think), and, to be honest, the engine braking is barely noticeable. I doubt that's because they leave the inlet valves wide open continually (e.g. when running downhill but not braking), as I don't believe that's possible in the Prius engine, but it certainly doesn't seem to have a lot of braking effect.

So, even if it was stopped, under regen, wouldn't the ICE would start to turn slowly, then once it got enough speed, the compression braking would be ineffectual, because (a) the actual compression phase is shorter in an Atkinson cycle engine, and (b) the rotational inertia would help it get over each compression stroke and into the expansion stroke, so the only significant losses would be frictional, with a few pumping losses thrown in???

Yeah, I can't buy that one, I'm sorry. The complexities are unthinkable. There's enough to do just synchroning the speeds of MG1, the ICE and the drive shaft when starting while already rolling, to minimise the jerk that would otherwise result. What has to happen, when starting the ICE while rolling, is that MG1 has to either slow down or even start turning backwards to get the ICE turning forwards at starting speed, and MG2 has deliver compensating additional torque to negate what's happening between the ICE and MG1, while together, their speeds match the current Power Splitter output speed. That's fairly complex, in itself.

There's no need to disconnect the ICE, so why would they do that? Then, one of the three inputs to the planetary gear set (Power Splitter) would be free wheeling as well, and there would be no drive. They need to prevent the possibility of freewheeling, which is why I believe there must be some kind of ICE brake.

 

Hi Jimbo,

Is that why the ORNL report constantly refers to MG1 as "Generator"? Because the only time it's really a motor is when it's starting the ICE? And ALL the regen power is generated and extracted from MG2, while MG1 is only generating when the ICE is running, so that the rest of the time it's more or less just freewheeling or acting as a brake (by being a generator) so that the ICE power actually gets delivered through the Power Splitter? That makes a lot more sense, but completely alters the way I thought it worked!

 

Thanks Patrick,

I knew you'd have the answer!


Yes, I'd already read the Wikipedia entry, but at the time (it may have been updated since), didn't "get" that MG1 wasn't involved when the ICE is off, which now appears to be the case. It has to be almost entirely freewheeling, or the ICE would start to turn due to the reactive torque. I think the gear ratio from MG1 to ICE is 3.6:1?

I guess MG1 may also act as a motor when the road speeds are high enough and the ICE is running but the load is low, to allow the ICE to slow down in order to save fuel. The MFD Power display seems to show a lot of swapping back and forth under those conditions, presumably to keep the HV battery "floating", neither over- nor undercharged.

 

Hi Greg,

That's what I was asking, but apparently, there isn't. MG1 apparently doesn't deliver/require any (significant) torque when the ICE is off. It's effectively just freewheeling, I guess intentionally so that the ICE DOESN'T get rotated, either forward to backward. With MG1 in "Neutral" there's no reactive torque to drive the ICE crankshaft.

The Prius design is just so amazing!

 

When I think of it like this, I see there's a good argument for inserting a simple dog clutch between the drive shaft and the Power Splitter. That would increase efficiency by allowing the Power Splitter and MG1 to stop turning on deceleration and downhill runs. I guess this will come when the Traction Battery gets bigger, and can drive the car for longer at highway speeds.

 

Indeed! But I was thinking that MG1 would be involved in Regen, because it's spinning much faster than MG2 and can therefore generate power at lower road speeds. But to achieve that, the ICE would have to be braked... Hence, my misconception in the first place...