jellydonut
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- Jan 9, 2011
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Perkins EnginesEngine Genetics
Engine performance for the genetically curious or engine genetics for the performance curious ...
The Eureka moment comes from an understanding of the relationship between mean piston speed and brake mean effective pressure (BMEP). If you don't know what these terms are, open an engine textbook and find out. For the terminally lazy, consider that mean piston speed is the average speed of the piston as it travels from top dead centre to bottom dead centre. BMEP can be considered as specific power per unit swept volume per unit rotational speed.
- Why is it that your cultivator engine plods along at a mind numbingly 3200 rev/min?
- Why does your average gasoline passenger car engine run to 6000 rev/min?
- Why are passenger car diesel engines restricted to 4500 rev/min?
- What's Montoya's BMW engine got that gives him 19000 rev/min?
- Is there a link between all reciprocating engines and are they 'brothers under the skin'?
Plotting BMEP against mean piston speed we can see:-
The Heavy Duty (HD) diesel engine is limited to a mean piston speed of about 11.5 m/s irrespective of bore and stroke. The data includes 400 kW on-highway truck engines and 4000 kW generating set prime movers. This is essentially a limitation on peak inlet port gas speed, characterised by mean piston speed. Air consumption is limited by the need to produce both an inlet swirl and flow, resulting in a mean inlet flow coefficient that is significantly worse than a gasoline engine. Since the diesel engine runs a significantly higher compression ratio, the peak cylinder pressures are higher than gasoline engines, the structures to support this loading are more robust. The diesel engine is, therefore limited to a lower mean piston speed than the gasoline counterpart by the ability to transfer air from the outside to the inside of the cylinder and by the higher friction losses associated with the higher mechanical loading.
The Light Duty (LD) passenger car engine runs to 13 -14 m/s mean piston speed as the lighter structures and reduced load factors contrive to lower the parasitic losses. Electronic fuel systems are limited to 4500 rev/min by the speed of the control solenoids and the pressures required to inject fuel into the cylinder in a short time period.
Development of the diesel engine proceeds parallel to the abscissa restrained by mechanical losses and to a lesser extent by air delivery issues. Ever higher boost ratios and mechanical loading limit the progress via material tribological and fuel system challenges.
A gasoline passenger car engine doesn't need swirl to mix fuel and air and runs to 18 m/s mean piston speed and slightly more in motorcycle configuration. The Honda S-2000 runs to 9000 rev/min, a staggering 23 m/s and would be the engine engineer's choice for 'engine of the decade'. Formula 1 engines run to 26 m/s aided by a design brief to achieve an advantage irrespective of cost and aided by uncompromisingly expensive and light components to limit inertia loading and a 25 hour life.
The Gasoline Direct Injection (GDI) engines run slightly higher mean piston speed than the port injected gasoline engine as the air consumption of the GDI is higher as the inlet system has only to pump air and not air + fuel.
Development of the gasoline engine proceeds at 60 degrees to the abscissa limited by parasitic losses, thermal loading and in the case of F1 engines, by valve gear durability.
It follows that:
Answers:
- All engines are brothers, related by mean piston speed and BMEP.
- The Honda S-2000 is the engineer's choice for engine of the decade but would your grandparents be happy with one?
- The diesel engine is capable of running faster than 4500 rev/min when fuel systems are capable. An engine speed of 10,000 rev/min is technically feasible.
- In the future, will turbocharging find favour on passenger car gasoline and will the gap between gasoline and diesel structures narrow?
- The limiting mean piston speed of industrial gasoline turf engines and passenger car engines are not dissimilar. The turf engine has a restrictive exhaust (higher parasitic loading) designed to minimise noise and keep the neighbours happy.
- The average gasoline runs to 6000 rev/min or 18 m/s mean piston speed as this is a happy balance between cost, rated speed power and the ability to sit at 70 mph at 2500 rev/min in top and still accelerate (just) with peak torque only some 1000 rev/min higher.
- The diesel engine is restricted to 4500 rev/min by the ability of the fuel system to deliver fuel at the correct quantity and at the correct rate to ensure complete combustion. Higher engine speeds are possible with continued fuel system development.
- Montoya's racecar engine runs to 26 m/s or 19000 rev/min as the only mechanical development restraint on this engine is the ability of the valve gear to stay in one piece.
- "Is there a common link between all engines?' …... mean piston speed and BMEP.
This article is somewhat old, considering the '1999 diesel' in the chart, and the mention of solenoid injectors which are 'old hat' today.
Still, a lot of good information that is valid for all engines.
I've seen so much of this 'diesels can't rev fast because of the slower combustion rate of diesel' or 'diesels can't rev fast because of the slower flame front' crap, it just needs debunking.
Diesel burns faster than gasoline, given proper air/fuel mixing. Diesel engines do not have a flame front - combustion occurs simultaneously at multiple points in the combustion chamber rather than spreading from a single point of ignition.
Main limitations of diesel engine speed:
- Intake and head structures that promote swirl lower the engine's volumetric efficiency, its 'ability to breathe'.
- Air/fuel mixture and efficient combustion (the reason engine designers try to achieve swirl in the first place)
- Injector firing rate.
- Friction losses associated with high compression ratios
- Heavy engine components associated with high compression ratios
Mazda has started to overcome many of these issues in their Skyactiv-D diesel engine design, an engine with the very low compression ratio of 14:1, associated low friction losses and low-weight components, and injectors capable of firing up to 9 times per combustion event (as opposed to the state of the art which is 4+1).
As you can hear, it pretty much sounds like a gasoline inline-four:
[ame="http://www.youtube.com/watch?v=PSEiK74yXlM&hd=1"]http://www.youtube.com/watch?v=PSEiK74yXlM&hd=1[/ame]
[ame="http://www.youtube.com/watch?v=xE9KDCnnhR8&hd=1"]http://www.youtube.com/watch?v=xE9KDCnnhR8&hd=1[/ame]
I am not at the stage where I can write articles about diesel performance and actually add anything useful to the world.