engine braking

And anyone with a clue still does it. Try riding a sequential-shift motorcycle by doing block shifting, for example.

That's just a feature/"weakness" of a sequential shift, nothing to do with having a clue.

Tim

You have *ALL* confused vehicle drag, transmission and engine friction loss with "engine braking". Diesels really don't have any appreciable engine braking.

Diesels just move a large volume of air though the engine. Suck doesn't take much power as there is no throttle. Squeeze it compresses the air. The inlet is open until about 30degrees ~1/8th stroke so volume of air passed though engine is about 7/8 of capacity. No fuel so no bang but expansion pushes piston back down bore, recovers nearly all the energy from compression, exhaust valve opens a bit before BDC. It's it's earlier than the inlet shut a bit more power will be used but usually timing is quite symmetric. Blow moves the air out of the engine at about atmospheric pressure.

Power taken to move that air is mass flow rate x (P exhaust - P inlet). Without the large volume of exhaust gas due to expansion from heat of combustion the exhaust has very little back pressure so P exhaust is close to P inlet. Power = mass flow rate x virtually NIL = not a lot.

Everything else you experience is engine friction and transmission losses which are about the same for both petrol and Diesel vehicles. Compared to the engine friction and transmission losses, the Diesel engine braking is naff all.

Petrol Sucks a 20 inHg / 0.67bar vacuum. The piston has move down against the force P x area pulling it up. Power taken is cylinders x 1/2 x force x area x mean speed. Squeeze, not a lot didn't manage to suck much but as valve is open to about 1/8 stroke it reduces the effect of the suck. So only 7/8 of inlet stroke counts for suction. Again no bang so it just expands what it squished. But it was unable to fill the cylinder on intake so at the bottom of the stroke it's pulling vacuum and it takes more power to move the piston down. But exhaust opens before BDC. Blow, the little bit of air inhaled gets expelled but as it's got vacuum it may suck air in from exhaust as the valve open and then blow it back out.

OK at 70mph the other drag forces cause about 15Kw of braking. An extra

No argument that it requires power just to shift air between intake and tailpipe.

That's right. Compression ratio does not of itself contribute to the engine braking effect.

Indeed.

OK. So can you explain why a two-stroke motorcycle, which will have a very low secondary compression ratio, has virtually no over-run braking when compared with a four-stroke motorcycle of similar capacity?

Chris

I haven't. Except a minor point that you correctly recognise that an engine has friction but say that the engine has no braking. You should qualify it by saying that "diesels don't have braking due to pumping".

Well done. I was trying to do some maths too but you saved me the trouble.

The other point about pumping loss is that such losses are greatest under cruising load - which is the driver behind direct injection. People have heard about throttle loss and made the mental connection that it is significant when closed. And then there are those that are stuck on compression ratios.

Even the diesel has a loss under load because the turbo is only 70% efficient and the intercooler adds a contribution. But this loss is significantly less than the benefit the turbo gives in reducing friction through lower running speed.

So removing the plugs doesn't make an engine turn more easily? Strange the things you're told after years of knowing different.

I get the impression there are lots of people here who've never driven a car. Or more likely shouldn't be allowed to.

The heat in the compressed air you dump down the exhaust also helps.

Except that you need to compare P at the inlet valve to Pat the exhaust = =

valve, they're not that close, even before you allow for valve timing.

Not if you try blowing it off with a 300W motor attached to a cylinder =

pump.

Actually they're not, the diesels got more friction

Nobody has claimed it doesn't allow an engine to be turned more easily by hand.

I get the impression that there are lots of people here that jump to the wrong conclusions.

At the same capacity, the two stroke will have lower friction. It is likely to have a higher output so to develop a higher bike top speed it might be geared for lower revs and the bike more aerodynamic.

You really don't have a clue.

Chris

So explain how that differs from it being turned by the movement of the car?

I get the impression bullshit rules here.

Satellite images.

What the f*ck are you talking about?

Of course diesels have engine braking. Maybe I'm imagining that the 20- odd diesels I've owned did it..

It's a fallacy, often repeated, that because diesels have no throttles they therefore have no engine braking.

Restrictions to airflow through a diesel engine occur in the inlet (much less than in a petrol engine) exhaust, particulate filter and to a great extent in the turbo, particularly in variable vane turbos. It's also not as if engines are frictionless, gearboxes ditto and pumping losses negligible.

Do you actually have diesel cars on your planet? I can only assume you've never actually driven one.

Tim

Desiel engines do not have engine breaking in the same sense that petrol engines do they restrict fuel flow to control power as apposed to airflow

Larger vehicles may have jake breaks which open the valves at the top of the stroke to vent of the power going to the crank , exhaust systems capable of being restricted to create a back pressure either via butterflies or closing off turbo vanes

. The jake brake is very effective but banned in many areas because of the noise it makes