Submitted by sepientr34 t3_10p6x3d in explainlikeimfive
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ScienceIsSexy420 t1_j6izx1y wrote
I know OP said turbine engine, but I'm pretty sure they that just meant a regular gas internal combustion.
TheJeeronian t1_j6j0qv7 wrote
Doubtful. Never seen a "regular" piston engine that comes anywhere near 10k revs.
MotoMatt5040 t1_j6j1gam wrote
You've gotta be kidding me? There's plenty that can rev higher. Motorcycles?? Even modded engines can rev to 14k+
ScienceIsSexy420 t1_j6j15e9 wrote
Hmmm, fair point. There are some very high performance gas piston motors that can do 9-9500, but unless they were looking st a Ferrari or a Shelby Mustang than you're right
HungryLikeTheWolf99 t1_j6j3748 wrote
Then there's the "similar fuel" comment. Diesel and jet fuel are more similar than diesel and gasoline.
ScienceIsSexy420 t1_j6j3odg wrote
Also true, but there are gasoline turbines (like the engine in the M1 Abrams)
TheJeeronian t1_j6jz0me wrote
Gas turbines can eat damn near anything, so it's not clear. Jet fuel is popular for jets but gasoline and diesel and natural gas are all common in gas turbines.
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.
CrushAtlas t1_j6iljum wrote
For one, the forces are different. You're comparing the rotational force of a turbine against the reciprocating mass of a piston engine, the forces at high RPM are incredibly strong on a connecting rod.
Secondly, diesel engines are compression/ignition design and are very efficient and make lots of torque at low RPM so there is no need to spin it faster.
sepientr34 OP t1_j6ilrae wrote
Why don't gas turbine have trouble burning fuel fast enough but diesel has (I never see sooth from gas turbine engine!)
Fred2718 t1_j6iqqah wrote
Older aircraft jet engines could be very smoky, especially at low altitudes. This was a problem for many reasons, e.g. military stealth is tough when the bad guy can see your smoke trail from 30 klicks. The one I'm familiar with is the CJ805 on the Convair 990 circa 1970, and you could see it on landing approach from 5 minutes away.
Sand_Trout t1_j6ioo90 wrote
Gas turbines can do a continuous burn to build pressure, limited only to the total flow of fuel and air.
Diesels are pulsed ignition, and are limited to how much fuel+air can fit in the cylinder without igniting early.
lowflier84 t1_j6ip1x4 wrote
Gas turbines produce soot, just like any other engine.
sepientr34 OP t1_j6ip5nw wrote
I know still do not as much as diesel engine
[deleted] t1_j6irffr wrote
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Leucippus1 t1_j6is1xk wrote
They are about as different as you can imagine other than the fact they use a similar fuel. Diesel and jet fuel are often interchangeable, that is true, in fact you can fill a Diamond Piston Diesel with jet fuel, that engine is based on a Mercedes 2 liter automobile engine.
A turbine is any device that is spun due to something pressing against a medium. A fluid turbine turns because a fluid passes over the vanes and moves the, a gas turbine does the same thing but with a gas. There isn't an articulating motion, the more pressure you send through the vanes the faster it spins and the more power you can create. The turbine is a specific part of the engine, the whole thing is called a turbine but if you plow a fan through a tube which has a rod along the long axis and at the end are some veins, that is also a turbine. Those big spinny things they build in fields are also turbines. In a gas turbine like you are talking about, you use compressor stages to compress the air before it hits the actual turbine. Everything spins along a shaft(s).
A diesel engine works by compressing an air/fuel mixture and combusting it such that it pressures the piston back down allowing another piston on the same crankshaft to rise to compress or exhaust. Assuming the combustions create more power than needed to keep the crank spinning you can attach a shaft to the crank and create spinning power.
Articulating devices are far harder on the metals than ones that simply spin, there are some diesels and gasoline engines that are capable of rotating at very high speeds but physics limits you. A turbine, on the other hand, only spins, and since you aren't sending something of mass out and back again (like a piston) your physical limits are a lot higher. Your reciprocating mass on a piston engine is far higher, the faster you go the more apparent mass on your components. A turbine will hit a limit as well, but they are at much higher RPMs.
SubmarineWipers t1_j6ixji4 wrote
If I recall correctly, there is something called roughly "ignition delay", meaning you need some time for the fine spray of fuel to start vaporizing AND physically "cracking" the long hydrocarbon chains by heat and pressure in the cylinder, before it can burn properly/ideally/equally everywhere.
This is possible to achieve under 4-4.5k rpm, but further increasing revs means there would not be enough time for this process during each combustion cycle.
I am sure my technical terms are wrong, I only read about it in my native language, so maybe someone will correct me, but a very smart guy was explaining it roughly like this.
TheDefected t1_j6j063k wrote
Petrol engine - air and fuel is already mixed, compressed, and then ignited with a spark.
Diesel engine - these aren't premixed, the engine has to take the air in, compress it, and at around the time the spark would ignite, that is when a diesel has to get in all the fuel it needs.
The faster an engine turns, the shorter that window too, so you tend to reach a point where there's not enough time to squirt enough fuel in and let it burn.
Modern diesels all tend to be common rail high pressure injection, usually around 1600-2000 bar. Having that sort of pressure means you can get a lot of fuel in quickly, but it still is a struggle.
I would guess the Audi LeMans diesel GT car would run at pretty high pressures, and also since it has a lot more cylinders, it doesn't need a massive bucketload in each one.
Gas turbines constantly add fuel and air, so it doesn't have the limitation of a small window to do everything.
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HungryLikeTheWolf99 t1_j6io62r wrote
The most important difference is reciprocating mass. Piston engines have reciprocating masses - things that move one way, then turn around and move the opposite way, all along the same axis. In a turbine engine, it just goes round and round, so nothing has to handle all the force of repeatedly reversing direction. It's also called an "engine", but it works entirely differently, and so very different physical limitations apply.
You're getting other answers that are sort of fixating on the "diesel" part of your question and why diesel engines rev a little lower than gasoline engines, but that's not the critical difference between piston engines and turbine engines - it's all about the reciprocation.