Lack of Engine Braking

JP08

Car is a 2015 Toyota Auris 1.2 Touring Sport (manual). Has done exactly what we want from it, as Toyotas tend to, re running round Cambridgeshire, up and down the A1 etc.

However, recently come back from driving round in the Yorkshire dales. And the lack of engine braking on descents is distinctly noticeable.

I'm wondering if its mainly due to

a) something to do with the engine working in an Atkinson Cycle mode under low loads, as, as I understand it, this means the valves are open longer and therefore less compression / vacuum in the cylinder ?
http://blog.toyota.co.uk/new-toyota-1-2t-engine

b) the sheer weight of the car compared to the small engine friction / losses ?

c) something else !?!

Ie is this something peculiar to the Toyota semi Atkinson cycle engines or does it plague other small engine big cars ?

Either way, descending the whole of length of the Swaledale side of Buttertubs Pass on the brakes felt wrong - tried engine braking but by the time we hit 5000 rpm and still accelerating in 2nd that felt and sounded wrong !

The Yorkshire trip also threw up another little foible with these small turboed engined cars. Stopping to give way going up a 1:6 means a slow subsequent climb in 1st until the gradient eases - unless you red line it and do a very swift gearchange the revs will have dropped out of the turbo zone before you get into second and then there is no go.

Strider590

Engine braking comes from the vacuum created by the throttle butterfly valve being closed, in some cars it is more closed at idle than in other cars.

It may be that in order to increase apparent fuel economy, Toyota have fitted some sort of throttle bypass to allow air into the engine when it would normally be in engine braking conditions.

Of course this is cheating, as it will increase wear on brakes and cost more in the long run, but most customers only care about hitting the specified MPG figures. Also a lot of drivers seem to dip the clutch when braking, so they probably won't even notice.

agrinnall

I'm going to have to experiment now, but I think you may be right: I think that the engine braking effect is less in my 1.0 Ecoboost Fiesta compared to my previous normally aspirated 1.25 engine, and as a result I use brakes more on a descent than I would have in the past. I don't think in my case it's a particularly significant difference, but perhaps if you've come from a car with a very different engine/weight configuration you might notice it more.

BeenThroughItAll

Strider590 wrote: »

Engine braking comes from the vacuum created by the throttle butterfly valve being closed, in some cars it is more closed at idle than in other cars.

ETA: I may be wrong about the below - I've been thinking about it more and I've managed to convince myself I just might be talking out of my backside.

I'm sorry, but that's rubbish. Engine braking comes primarily from the effect of compression and mechanical load without fuel being burned - the vehicle's wheels turning the engine against its own compression as the resisting force, rather than the engine turning the vehicle's wheels through combustion.

Vacuum in the manifold has little or nothing to do with it - diesels don't create a vacuum in the inlet, so how would you experience engine braking in a diesel with your theory?

DoaM

BeenThroughItAll wrote: »

I'm sorry, but that's rubbish. Engine braking comes primarily from the effect of compression and mechanical load without fuel being burned - the vehicle's wheels turning the engine against its own compression as the resisting force, rather than the engine turning the vehicle's wheels through combustion.

Vacuum in the manifold has little or nothing to do with it - diesels don't create a vacuum in the inlet, so how would you experience engine braking in a diesel with your theory?

I'm guessing the info came from Wiki?

Diesel engines apparently use a different mechanism to achieve the same end.

JP08

Both of you could be right. To quote from the link about the Toyota engine in my original post.

"From Otto to Atkinson
The VVT-i system operates on both the intake and the exhaust side, and allows torque to be maximised at all engine speeds. In addition, the VVT-iW allows for intake valve closing to be delayed, which means the engine can operate in both the Otto and Atkinson cycle.
The Atkinson cycle is used in extremely low engine load conditions, when the intake valve remains open for a fraction of time, after the compression stroke has set in, allowing part of the gas charge to be pushed back into the intake. As a result, the effective compression stroke is shortened.
Pumping losses are reduced, as there is less pressure on the piston, and the throttle valve can be opened wider."


Though I still think the weight for engine capacity plays an equally significant part.

Mercdriver

I am not mechanically minded, so wouldn't know who was right or wrong. But when offering solutions, it might be better if people gave the source of their knowledge even if it is a self editing source like Wiki.

I wouldn't even begin to offer advice on such things, Strider's advice is often off kilter, so it would be useful if Strider would tell us where he got this fount of all knowledge from, if it was from Wiki, he should have said so.

BeenThroughItAll

DoaM wrote: »

I'm guessing the info came from Wiki?

Diesel engines apparently use a different mechanism to achieve the same end.

No, the information comes from 30 years of understanding how an engine works, I'm afraid, but Wikipedia should reflect the same information.

ETA: Unless I don't understand and I'm wrong as I've said below.

BeenThroughItAll

JP08 wrote: »

Both of you could be right. To quote from the link about the Toyota engine in my original post.

"From Otto to Atkinson
The VVT-i system operates on both the intake and the exhaust side, and allows torque to be maximised at all engine speeds. In addition, the VVT-iW allows for intake valve closing to be delayed, which means the engine can operate in both the Otto and Atkinson cycle.
The Atkinson cycle is used in extremely low engine load conditions, when the intake valve remains open for a fraction of time, after the compression stroke has set in, allowing part of the gas charge to be pushed back into the intake. As a result, the effective compression stroke is shortened.
Pumping losses are reduced, as there is less pressure on the piston, and the throttle valve can be opened wider."


Though I still think the weight for engine capacity plays an equally significant part.

Well the Atkinson cycle engine does run a different cycle, with valve openings at different periods, which has a knock on impact on the overall compression when turning over the engine. The throttle valve opening thing is a simple red-herring; compression and mechanical friction are the major root of engine braking.

If you don't believe me, try turning the engine off and coasting in a car with your foot on the accelerator, and then again with it off the accelerator - without the fuel being ignited, you'll see almost no difference in the deceleration caused by the engine braking alone.

ETA: You know, the more I consider this, the more I wonder if I'm right. The closed throttle valve on a petrol engine would theoretically mean the engine is unable to draw in air on the inlet stroke, which would mean harder work in that part of the cycle. The compression stroke would compress the small amount of air in the cylinder, resulting in engine braking in that part of the cycle. In theory a lot of that effort would be returned in the power stroke as the compressed air expands against the piston, with the exhaust stroke only generating any braking as created by the back pressure in the exhaust system.

So I may well be wrong now I think about it - but that would mean the 'no engine braking' effect suggested for diesels would be at odds with all the training I've done off-road, in which the use of engine braking is a key part of descending steep inclines, and one for which diesel engined vehicles are always suggested as being 'better' at.

So ignore me for now. I'm old enough and ugly enough to accept I could well be completely wrong.

EssexExile

Well I was very impressed with all that vacuum & throttle stuff & thought I was learning something new at my advanced age. Then you go & tell me it's compression, which is what I'd always assumed it was, so now I'm disappointed I haven't learnt something new but happy I was right all along.

Drive a traditional automatic the you get used to little or no engine breaking.

BeenThroughItAll

EssexExile wrote: »

Well I was very impressed with all that vacuum & throttle stuff & thought I was learning something new at my advanced age. Then you go & tell me it's compression, which is what I'd always assumed it was, so now I'm disappointed I haven't learnt something new but happy I was right all along.

Drive a traditional automatic the you get used to little or no engine breaking.

Well I was like you, assuming it was compression which is what I've always 'known' it was. But the more I thought about it, the more I've managed to convince myself that inlet tract vacuum may have a more significant part to play than I believed.

Still not convinced on the diesel front though.