You get some of it back on the expansion stroke, but as the valves don't open at the optimum times for this a chunk of the stored energy just ends up wasted.
Yes, so the heat that leaks into the cylinder walls etc. before the refrigeration kicks in, is the only part of the heat generated that actually contributes to engine braking.
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.
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Mass flow rate of 1.9L Diesel at 4000 rpm is 0.5 x 7/8 x density kg/m³ x volume m³ x rpm / 60 = 0.5 x 7/8 x 1.22 x 1.9 x 10^-3 x 4000 / 60 = 0.0676 Kg/sec. Maybe 1 inHg back pressure Pexh - Pin = 3386 pascal. Power consumed = 0.0612 x 3386 = 229 Watts. The rice pudding isn't at any risk of losing it's skin.
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.
4 cylinder 1.8L petrol, bore 83mm, stroke 83.6mm at 4000rpm. Power consumed = cylinders x 0.5 x vacuum (pa) x area x 0.75 x stroke x rpm / 60 = 4 x 0.5 x 67727 x 0.083^2 x pi/4 x 0.75 x 0.0836 x 4000 / 60 = 3063 Watts = 3 Kw. This is quite pessimistic as I haven't included the suction at bottom of power stroke or reverse filling and pumping of exhaust - it won't amount to much different than a Diesel.
OK at 70mph the other drag forces cause about 15Kw of braking. An extra
3Kw is only 20% more, not a hugely noticeable difference. At 20mph in
1st you will find it's much more significant.
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?
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.
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.
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.
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
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