dr_xenon t1_j6imocs wrote
A Diesel engine, or any piston engine is limited by the linear speed of the piston. When the fuel air mixture is ignited, the flame front travels around 4000 ft/min. The piston can’t travel any faster than that. The stroke of the engine and the rpm determine how fast the piston is going.
An engine with a short stroke can rev much higher than one with a long stroke. A diesel engine tends to have a very long stroke to get the high compression ratios, hence a low redline. A small motorcycle engine has a short stroke and can rev much higher.
Rotary/wankel engines don’t follow this rule.
Turbines have a different combustion system in stages which allows it to go higher rpm.
pseudopad t1_j6io30s wrote
I have a very rudimentary understanding of engines and this laid out very clearly why engines revolve at the rates they do. Thank you.
As a side note, do you know why that one diesel le mans car from Audi could still rev as high as it does (8k iirc)? Is it kind of the same reason as why high performance gas engine also rev much higher than regular gas engines.
RampantRooster t1_j6iu5uy wrote
Generally an engine will encounter issues with reciprocating weight and getting enough fuel and air before it starts seeing issues with reaching the speed of the flame front. Racing engines have different requirements than road engines that allow different construction methods to reach higher RPM.
They can use lighter and possibly more expensive internals. This is bad for road engines because they have a lower lifetime and are more expensive to fix, but fine for racing cause they rebuild engines often. There's also more advanced valves operated pneumatically or electrically that maximize their open duration and minimize the time it takes to open or close them. Again these are more expensive to maintain, which is fine for racing but bad for road vehicles.
ELI5 summary: throwing more money at specific engine components can increase redline, but doing so is bad for regular car engines due to maintenance costs and reliability
I'm not familiar with that particular engine, but an example of a road diesel engine that has a redline above 5k RPM is the Mercedes OM606. It has dual overhead cams and pistons with a larger bore than stroke, which isn't the norm for diesels.
Fred2718 t1_j6ipwld wrote
Recip engines of various sizes are limited also by the maximum acceleration of their parts. More precisely, by the maximum strain energy per unit volume in critical parts, like the rod bearings. McMahon's book On Size And Life has a very good description of this and an analysis of engines ranging from tiny model airplane motors running at 25000 rpm, to gigantic industrial motors run ing at a few tens of rpm.
sepientr34 OP t1_j6in4vz wrote
If I remember correctly flame from turbine need to be ignited once than you just switch of the ignition unlike piston engine
RadBadTad t1_j6iv831 wrote
Yes, it's like a pilot light on your water heater or oven. Once it's lit, it remains lit, as there is one long continuous combustion rather than repeated small explosions like in an internal combustion engine with pistons.
/* With turbines, it's actually that once they get up to self sustaining speeds of rotation, the compression they achieve is enough to ignite any fuel that is sprayed into the combustion chamber, which then expands and turns the blades on the way out of the engine, which continues to turn the compression blades up front, which maintains (or ads to) the speed of rotation. It's actually a very neat process to me.
The way they start up multi-turbine aircraft is basically by hooking up a small portable turbine that pushes air through the first engine on the aircraft, which begins turning the compressor on that engine until it achieves ignition and becomes self sustaining. Then, they disconnect the portable engine, and close some vents in the running engine and shunt the spare airflow to the next stopped engine, which gets that one turning until IT achieves sustaining speeds, and so on.
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