You can do that, but why make the engine heavier than it need be? To get power to weight, you either have to up the peak pressure in the cylinder (higher compression ration or forced induction) or rev the thing more. Or simply have a bigger enginbe. To change the torque so it moves lower down, you use a longer stroke smaller cylinder, but then you lose power..and top end RPM limits..
It can all be done, but the upshot is that manufacturers build to what most people want.
Well that's the point. you can always 'get more' with 'bigger, and therefore heavier' But the trick is to 'get more' with the same amount of frictional losses and weight.
Same CC and longer stroke means better low down torque BUT less top end power.
You can then blow that and get even more low down torque, but still be stuck with no RPM at the top.
Which is essentially what a turbo diesel is.
But its a heavier option than a revvy petrol engine. For the same power.
No. Why would it? Its easier, but not a feature of it. To get high compression ratios on a short stroke engine means a very tight tolerance on things like head gaskets, but lots of racing engines manage OK.,
Its the same pressure operating over a longer stroke so its connected to a bigger crank, so there is more leverage on the crank for the same cylinder pressure. I.e. more torque for a given cylinder area. Of course the actual force is reduced due to the cylinder AREA being less for the same displacement. There is a complex interplay with engine frictions as well. But I wont go into that because its all so long ago I have forgotten it. ;-)_
But overall there is a slight gain and long stroke engines simply work better at low RPM but don't develop the same power as short strokers because they cant rev so much ..
And they generally have better low down torque because they can be optimised to work there.
No. Diesel is a very fast burn. It is a knock. You get very fast burn,. higher compression rations and higher peak pressures in a diesel, and to develop this its normal to use a long stroke smaller bore engine with a lower rev limit.
All this of course is becoming rather irrelevant with things like variable valve timing and variable geometry turbos.
You can optimise the breathing over a much higher rev range.
Modern engine design owes almost nothing to the traditional arts. A good BMW turbo diesel or a VW turbo petrol are simply miles away from the BMC A series I grew up with ;-)
Or even the turbos of the 80s and 90s which were my first blown cars.
The old rules of tihumb really don't apply with modern cars, or apply far less.
It used to be a mechanical compromise between compression ratios, breathing, valve timing ignition timimg and carburettor mixture ratios etc etc..
EFI and electronic ignition, variable valve timing and supercharger or turbochargers made it all redundant. And now variable geometry turbo chargers to eliminate the need for tow or three stages..
The engines today bear little resemblance to those of even 20 years ago. And te old 'rules' seem to no longer apply.
Ftankly I have driven woefully torqueless diesels and very torquey petrols of similar capacity, and once you stick a turbo on, all bets are off.
On my current TD, the turbo cuts in at ridiculously low RPM and gives massive low down torque. And it keeps going to 4K RPM plus. very wide power band
1 The pressure is the same, independent of stroke.
2 The same volume is swept.
Yet you say the torque is greater for a longer stroke, even when the area is reduced by the same ratio.
Given the energy per stroke is independent of area and stroke, ie dependent on the swept volume, perhaps you can explain where this extra torque comes from?
Does that mean the most efficient engines are ones with long strokes? Why do you think the most efficient engines are of the "square" variety?
Well, yes. Due to the reduced area available to valve arrangements one would expect a long stroke be asthmatic so appear better at lower revs.
One of the main factors is compression ratio. At higher compression ratios, fuel is burns faster and more completely (hence more efficiently) This is one of the reasons why a diesel engine uses less fuel. (Much higher compression ratio)>
So why don't we see high revving engines on the same scale as F1 engines, say 18,000 rpm?
There's me thinking that a diesel at full chat was about the same efficiency as a petrol engine!
Reasons for greater economy: Isn't diesel denser fuel in terms of joules/kg? I also thought the higher efficiency was in part due to minimal induction losses at part load / throttle?
Actually the new turbos are running more down around th 12-15k these days I think.
The answer actually lies in the ability to work a valve at those sorts of RPM F1 engines used to use compressed air to slam the valves shut as springs wouldn't cut it.
If the engine is small enough and the valves are rotary, you can get a lot higher that 18K RPM..
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Wait till the tuned pipe comes on song and laugh at the sheer stupidity of it all.
It depends on where you allow 'full chat' to be Diesel takes in constant air, and that lean burns to get peak efficuiency at lower powers.
Ex of blowing as power demand goes up, you inject more fuel so peak throttle and power is often overfuelled and spewing black smoke as a result. Blown diesel are able to add more air as well, and you may limit the fuel so they are producing less power than they might, but with decent emissions and efficiency.
Also the higher cylinder pressures in a diesel make it marginally more efficient for complex reasons, and the generally lower RPM reduces frictional losses too.
Petrol tends to be most efficient flat out, and may well approach or exceed diesel flat out efficiencies, but cars spend more time at part throttle.
I think so. Not sure.
Diesel is VERY lean burn at part throttle
As I said all the old rules are bolixed by EFI and by turbocharging., You can produce wildly different performances out of the same mechanical parts by varying the software alone.
I.e a relatively long stroke low RPM small engine will have very low frictional losses and can be very economical if you can control the lean burn. It will also be crap for peak power...until you pop a turbo on it, when suddenly its producing mammoth amounts of torque, and if its a variable geometry turbo, massive torque from well down the rpm range too.
I think todays engineers have so much freedom of design that they can with EFI and turbos produce pretty much whatever sort of engine cures they think people will like.
Power is related to the pressure difference the charge expands through but the diesel, despite having higher compression ratio, doesn't burn a stoichiometric mixture, it's always slightly lean, so less heat to do the work and also as it burns slower, because of the duration of the injection the fuel at the end of the injection is applying heat which has less time to do work through a lower pressure drop, less heat is converted to motion. This is where a common rail diesel scores, the faster injection and much finer droplets mean the burn is completed more quickly.
The diesel really scores at part throttle as it always expands a full charge of air from the same initial pressure, albeit it applies less heat. The SI engine throttles the charge so the pressure it expands from is less.
About the same but the diesel is 10% more energy per litre, and you pay per litre
So we're agreed on this, especially when taking into account the denser nature of diesel.
This was the original and erroneous point you made earlier which I felt obliged to question and repeated here. You chose not to answer the point I raised in an earlier reply.
Long stroked engines are confined to the scrap heap. Long stroke engines only came about because of tax laws governing HP ratings which only depended on cylinder area and not stroke.
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Long stroke engines have more friction, for the same swept volume the pistons have a circumference which does not reduce proportionately with stroke. So overall a greater area of skirt x distance travelled.
Piston friction is not the only source of friction.And friction is a function of load applied to the bearing surfaces approximately.And all other things are NOT equal. You need to be pretty good in ring seal on a short stroke engine and that increases friction.
Otherwise we would go to massively oversquare engines, and we simply don't. There is an optimal range of bore/stroke ratios. And you get different characteristics from them.
AS I said, its all complicated and the devil is in the details. Modern diesels tend to be under square and petrols over square. Why?
Modern petrol engines tend to be less oversquare than once was the case. IIRC, due to emissions. Difficult to meet them with a very oversquare design. Also one reason why 4 valves per cylinder is the norm - to get an adequate area with a smaller combustion chamber.
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