Why can't diesels rev..
Discussion
Diesels can rev, if you've ever seen a diesel runaway by sucking up the engine oil through the crankcase ventilation system and combusting it, they'll rev until they chuck a conrod through the block! (Heard a story of one revving to 9000 rpm before going bang)
Diesels DON'T rev for many reasons:
Higher reciprocating forces due to high piston and conrod masses. (Although generally you would find that the gas forces are the biggest killer i.e. peak cylinder pressures of 180 bar versus roughly half that in gasoline)
Diesels don't breath very well at high revs. The valves are perpendicular to the cylinder head flame face because of the piston top face at TDC is only a fraction of a millimeter from the flame deck. This is because you have to make sure the combustion is only in the piston bowl by eliminating any crevice volumes. There is very little scope increasing valve area (via a pent-roof) or introducing variable valve timing. Swirl inducing ports are also very bad for volumetric efficiency aswell.
The biggest reason though is combustion duration. Diesel combustion is like the yellow flame on a bunsen burner (whereas gasoline is like a blue flame on a burner because the air and fuel is premixed or homogenous). This yellow flame is called a diffusion flame. Combustion occurs at the fuel surface where the air and fuel are next to each other. This is very slow. To speed this up you can mix up the air fuel by introducing turbulence (i.e. swirl) and/or you can increase the fuels surface-area to volume by increasing injection pressure. Problem is heavily turbulent combustion is not very controlled and emissions are therefore high. Increasing injection pressure has it's own mechanical challanges like sealing, fuel pump wear, parasitic losses and too many other reasons to mention.
A very brief overview, thousands of people have PHds into each tiny aspect of what goes on above!
Cheers
Phil
>> Edited by wheeljack888 on Wednesday 7th September 17:55
Diesels DON'T rev for many reasons:
Higher reciprocating forces due to high piston and conrod masses. (Although generally you would find that the gas forces are the biggest killer i.e. peak cylinder pressures of 180 bar versus roughly half that in gasoline)
Diesels don't breath very well at high revs. The valves are perpendicular to the cylinder head flame face because of the piston top face at TDC is only a fraction of a millimeter from the flame deck. This is because you have to make sure the combustion is only in the piston bowl by eliminating any crevice volumes. There is very little scope increasing valve area (via a pent-roof) or introducing variable valve timing. Swirl inducing ports are also very bad for volumetric efficiency aswell.
The biggest reason though is combustion duration. Diesel combustion is like the yellow flame on a bunsen burner (whereas gasoline is like a blue flame on a burner because the air and fuel is premixed or homogenous). This yellow flame is called a diffusion flame. Combustion occurs at the fuel surface where the air and fuel are next to each other. This is very slow. To speed this up you can mix up the air fuel by introducing turbulence (i.e. swirl) and/or you can increase the fuels surface-area to volume by increasing injection pressure. Problem is heavily turbulent combustion is not very controlled and emissions are therefore high. Increasing injection pressure has it's own mechanical challanges like sealing, fuel pump wear, parasitic losses and too many other reasons to mention.
A very brief overview, thousands of people have PHds into each tiny aspect of what goes on above!
Cheers
Phil
>> Edited by wheeljack888 on Wednesday 7th September 17:55
To put the above post in laymans terms, the bang in the cylinder is'nt so much of bang more of whoosh compared to petrol. Hence beyond say 5k rpm the whoosh is only just finished when the piston gets to the bottom & any faster with the pistons coming back round the whoosh will turn into crash bang wallop like TVR's do but different 
thong said:
jet engines are diesels and they rev
No, they're not.
There is however a variety of gas turbine which uses a diesel engine instead of a continuous combustion chamber as a source of hot gas. The engine in question has no crankshaft or con rods; it's just a pair of opposed pistons bouncing back and forth in a tube, with a compressed air spring taking the place of the flywheel action. It produces no mechanical power output, and is basically just a complicated way of burning fuel. The mechanical output comes from the turbine in the exhaust.
Pigeon said:
jet engines are diesels and they rev
No, they're not.
There is however a variety of gas turbine which uses a diesel engine instead of a continuous combustion chamber as a source of hot gas. The engine in question has no crankshaft or con rods; it's just a pair of opposed pistons bouncing back and forth in a tube, with a compressed air spring taking the place of the flywheel action. It produces no mechanical power output, and is basically just a complicated way of burning fuel. The mechanical output comes from the turbine in the exhaust.
Sounds very interesting - What is the reasoning behind that unusual appraoch? (I'm intrigued)
stig said:
Depends on the size of the diesel. R/C model engines are diesel and typically rev to 30k and beyond!
As said above, burn-time and reciprocating mass all play a part.
You mean some are Diesels. Glow engines are more a sort of semi-Diesel, having a glow plug with a platinum element which starts combustion catalytically. Model Diesels don't have injectors either, they have carburettors and the mixture ignites when it's hot enough (which, apparently, the timing is surprisingly accurate). Compression is adjustable which varies the timing. They aren't as revvy as glow engines, with top whack being more like 11-25k depending on the engine, www.eifflaender.com/techdets.htm with some glow engines revving to perhaps 40k. But model engines aren't constrained by emission limits (yet).
wheeljack888 said:
Diesels can rev, if you've ever seen a diesel runaway by sucking up the engine oil through the crankcase ventilation system and combusting it, they'll rev until they chuck a conrod through the block! (Heard a story of one revving to 9000 rpm before going bang)
Saw this watching Scrapheap Challenge: Scrappy Races the othert day!
One team had a tractor/truck thing with a 16 litre truck engine in it. They were trying to increase the power output by adjusting the governor to allow more revs. They guy doing the tweaking went a little too far and the adjustment screw came out completely and fell off. Cue one runanway engine....
They managed to stop it by cutting off the air supply. But not before it had snapped several pushrods and wrecked the bottom end.
I should thnk that the main reason that Diesels don't rev, is because you cant control when combustion occurs as tightly as with a spark ignition engine.
Obviously with compression ignition, the compression is more or less the same throughout the rev range, but the piston speeds are proportional.
Because the flame ignites at the same (ish) compression pressure in each stroke, and the flame burns at a fixed (ish) rate, the power of the combustion varies in relation to TDC, becuase the piston speed varies with engine speed.
As a consequence the point of power is always a compromise, between excessive pinking, and a late burn.
Model engines rev sky high, because they generally have adjustable compression. Once you get them started, you set the mixture, then the compression, and go back to the mixture, and then do the compression again. Ultimately the compression for the fastest engine speed is very different from the slowest one.
On the other hand it could be that Diesels don't rev, because they have pistons made of Potato!
>> Edited by dilbert on Thursday 8th September 17:18
Obviously with compression ignition, the compression is more or less the same throughout the rev range, but the piston speeds are proportional.
Because the flame ignites at the same (ish) compression pressure in each stroke, and the flame burns at a fixed (ish) rate, the power of the combustion varies in relation to TDC, becuase the piston speed varies with engine speed.
As a consequence the point of power is always a compromise, between excessive pinking, and a late burn.
Model engines rev sky high, because they generally have adjustable compression. Once you get them started, you set the mixture, then the compression, and go back to the mixture, and then do the compression again. Ultimately the compression for the fastest engine speed is very different from the slowest one.
On the other hand it could be that Diesels don't rev, because they have pistons made of Potato!
>> Edited by dilbert on Thursday 8th September 17:18
You can make a diesel rev like a bike engine by piping propane into the inlet plenum, try it! It's always a good idea to have an air cut off though!
Forget nitrous oxide, (which works with diesel) propane is the tool of champions. I had dealings with a diesel forklift which sniffed some propane. It went off the blocks like a hillclimb car!
Forget nitrous oxide, (which works with diesel) propane is the tool of champions. I had dealings with a diesel forklift which sniffed some propane. It went off the blocks like a hillclimb car!
It does have to be said that diesels are generally capable of revving higher than their governors are set to allow. But combustion becomes progressively less efficient, so you use a disproportionate amount of fuel for the power output, and kick out loads of black smoke.
One way of getting a diesel to rev higher is to revert to Uncle Rudi's original idea of using a blast of compressed air to inject the fuel, which gives much better fuel atomisation.
LM's propane trick is a well-known and rather neat method of getting more output from a diesel. Even at full power a diesel only uses about half the available oxygen in the air, because you simply can't get the injected fuel to mix thoroughly enough to use all of it without leaving a lot of unburned fuel. If you add some propane to the intake you can use this spare oxygen. Since propane is remarkably resistant to ignition (high octane rating) you don't get the massive advanced detonation you'd get if you tried this with petrol; it doesn't burn until you inject the diesel and the burning diesel ignites it.
It's basically an intermediate evolutionary stage... you start with an ordinary piston engine, then you turbocharge it, then you fit a bigger exhaust turbine that generates more power than the compressor side consumes and add the extra power to the power from the crank, then you ignore the crank and just take power from the turbine, then you do away with the crank and you get a free-piston engine, then you do away with the pistons and you get a gas turbine.
If you compare the free-piston diesel with an ordinary diesel providing power from the crank, it is impressive in terms of specific output and mechanical simplicity, and since a turbine can provide torque at zero revs you may not need a clutch in the transmission. However, the lack of any direct mechanical connection to the pistons makes it difficult to control and to start, and compared to a "straight" gas turbine it has lower specific output and is more complicated. Once metallurgy had advanced to the point where a "straight" gas turbine was no longer a flaky proposition, people largely lost interest in the free-piston engine.
They do however have a small niche in applications that can take power from the reciprocating motion of the pistons without need for a crank, so there are some about driving hydraulic pumps and pneumatic compressors; there are also some that deliver power electrically, by means of a magnet attached to the piston shuttling back and forth in a coil of wire (a "linear alternator"
.
One way of getting a diesel to rev higher is to revert to Uncle Rudi's original idea of using a blast of compressed air to inject the fuel, which gives much better fuel atomisation.
LM's propane trick is a well-known and rather neat method of getting more output from a diesel. Even at full power a diesel only uses about half the available oxygen in the air, because you simply can't get the injected fuel to mix thoroughly enough to use all of it without leaving a lot of unburned fuel. If you add some propane to the intake you can use this spare oxygen. Since propane is remarkably resistant to ignition (high octane rating) you don't get the massive advanced detonation you'd get if you tried this with petrol; it doesn't burn until you inject the diesel and the burning diesel ignites it.
Fatboy said:
There is however a variety of gas turbine which uses a diesel engine instead of a continuous combustion chamber as a source of hot gas. The engine in question has no crankshaft or con rods; it's just a pair of opposed pistons bouncing back and forth in a tube, with a compressed air spring taking the place of the flywheel action. It produces no mechanical power output, and is basically just a complicated way of burning fuel. The mechanical output comes from the turbine in the exhaust.
Sounds very interesting - What is the reasoning behind that unusual appraoch? (I'm intrigued)
It's basically an intermediate evolutionary stage... you start with an ordinary piston engine, then you turbocharge it, then you fit a bigger exhaust turbine that generates more power than the compressor side consumes and add the extra power to the power from the crank, then you ignore the crank and just take power from the turbine, then you do away with the crank and you get a free-piston engine, then you do away with the pistons and you get a gas turbine.
If you compare the free-piston diesel with an ordinary diesel providing power from the crank, it is impressive in terms of specific output and mechanical simplicity, and since a turbine can provide torque at zero revs you may not need a clutch in the transmission. However, the lack of any direct mechanical connection to the pistons makes it difficult to control and to start, and compared to a "straight" gas turbine it has lower specific output and is more complicated. Once metallurgy had advanced to the point where a "straight" gas turbine was no longer a flaky proposition, people largely lost interest in the free-piston engine.
They do however have a small niche in applications that can take power from the reciprocating motion of the pistons without need for a crank, so there are some about driving hydraulic pumps and pneumatic compressors; there are also some that deliver power electrically, by means of a magnet attached to the piston shuttling back and forth in a coil of wire (a "linear alternator"
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