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Diesel engines provide many advantages over gasoline ones. Because they opperate on the principal of autoignition, they have much higher compression ratio enabling higher efficiency. Diesel fuels higher energy content also improves fuel economy, and because it burns much more slowly than gasoline the
combustion exerts force on the crankshaft longer during each combustion stroke improving torque (one of the reasons diesel engines are stump pullers). However, they also have some disadvantages. To handle higher cylinder pressures, the components of a diesel engine must be made much heavier. Also, because diesel fuel is injected right at TDC, there is no time for it to mix with the air already in the cylinders leading to soot formation and incomplete combustion diesel engines are known for. This is also the main factor (in addition to longer burn time) that limits the maximum opperating speed of diesel engines.
What I propose is a duel-fuel, port and direct injected engine that improves combustion quality and retains diesel efficiency while not requiring heavy duty components. Atkinson cycle engines are designed to force an amount of air back out of the cylinder during the combustion stroke. This improves volumetric efficiency, as the expansion stroke is longer than the compression stroke allowing more energy to be harnessed during combustion. Also, because it is the expansion ratio and not the compression ratio that ultimately determines efficiency, an atkinson cycle with a final compression of, say, 10:1 but a mechanical compression ratio of 12:1 will have the efficiency of a 12:1 engine. The only disadvantage to this is that an engine of a given displacement will produce lower power than an equivalent non-Atkinson cycle engine because its actual displacement is what remains after some air is pushed back out during compression.
The engine would have high pressure (diesel fuel requires high pressure to atomize) port fuel injectors for diesel fuel and direct injection for ethanol. It would have a mechanical compression ratio of 15:1, nearly equivalent to a conventional diesel, and a maximum final compression of around 10:1. While this means that a 2.4L four cylinder engine would have an actual displacement of 1.6L, the fact that diesel fuel has an energy content of 30% greater than gasoline would put power output at around that of a 2.1L gasoline engine.
The diesel fuel that is port injected has ample time to mix and form a homogenous mixture with the air. Variable valve timing could delay intake valve closing to act as a throttle (the more delayed the valve closing, the more air that gets forced back out during compression). When the piston nears the top of the compression stroke, a very small amount of ethanol is injected surrounded the spark plug, as in conventional lean-burn direct injection engines. This facilitates spark ignition, and also aids in reducing pre-ignition tendency.
The engine would nominally run on almost completely diesel fuel in low load opperation, but if excessive knock is detected, the ratio could be altered to reduce it, with two knock-reducing advantages of the ethanol injection being its high octane and ability to absorb heat through evaporation.
The benefits of this engine would be an efficiency equal or greater than a conventional diesel (possibly increased due to Atkinson cycle), significantly lower emissions without the need for urea injection or soot filters, and no need for heavy componants (whose rotating mass decreases efficiency).
Spark-Ignited Compression Ignition Combustion
http://jcwinnie.biz/wordpress/?p=2697
[FlyingToaster, Jun 27 2011]
[link]
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Diesel engines have higher compression ratios because they can. If petrol engines could run at 20:1, they would. |
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Diesel fuel in an IC engine burns more quickly than petrol. In a petrol engine, ignition occurs at the spark plug and is then delayed by the rate of flame propogation from that point. In a diesel engine, ignition occurs throughout the charge almost simultaneously, giving the characteristic diesel 'knock'. Modern diesels use multipoint injection to smooth this out. High torque is the product of high BMEP. |
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However quick or slow the burn, pressure is still exerted on the piston until it is released by the exhaust valve. Arguably, a quicker burn might allow more loss of pressure due to cooling during the remainder of the stroke. |
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Fuel is injected at the moment most suited to the conditions (RPM, load, demand, charge temperature etc.) Sooting is the result of faulty injectors. The mixing of fuel and air by injection is superior to that by carburation. |
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Audi and Peugeot manage to run diesel engines in LMP cars at fairly high RPM. |
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The long explanation of why an IC engine is most effecient at less than full power (masquerading as an explanation of the Atkinson cycle) is amusing. |
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Pumping losses from keeping the inlet valve open to reduce compression would be significant, and probably greater than throttling losses to achieve the same effect. |
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Part of the effeciency of diesel engines comes from the fact that they are not throttled. The full volume of air is taken into the cylinder and the required volume of fuel is burned to expand that air to produce power. |
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Reducing compression will reduce combustion temperature and increase soot production. |
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The rotating mass of heavy components need not decrease effeciency. All car engines are fitted with flywheels, because the rotating components do not have enough momentum to ensure smooth running and RPM transitions. |
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In contrast to a bifuel diesel, where high-compression ignites the diesel, which high temperature in turn ignites the methane/propane, this uses a spark-ignited reaction to raise the pressure enough to detonate the diesel. |
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Probably easier to convert an existing engine than the other way'round. |
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I take it the Atkinson cycle is just a minor addition to the main idea ? |
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The Atkinson cycle is to allow the same expansion
ratio as a conventional diesel (hence efficiency) yet
keep the final compression low enough to prevent
preignition. |
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Direct fuel injection is definately a better way of
mixing air and fuel than carburation, but injecting
fuel when the piston is at TDC is certainly not as
efficient as port fuel injection; no matter how high
the pressure is the fact is that the fuel has no time
at all to mix with the air before combustion, which
is mostly what causes the soot. |
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Pumping losses would most certainly not be greater
than that of a throttle; it is easier for the engine to
intake air unrestricted and then force some out
than create lots of vaccuum via throttle. This is also
why more recent and advanced engines from some
car manufacturers (BMW and Nissan are a couple I
can think of off the top of my head) are using
variable valve lift to replace conventional throttles. |
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The Audi and Peugeot diesels' "relatively high RPM"
that you speak of is around 5,500 which really isnt
that high. This system would allow for easy
convertion of existing gasoline engines to diesel
and would mimic the characteristics of a gasoline
engine. Because the mixture would already be
homogenous and spark timing could be advanced, I
would predict a maximum opperating speed of at
least 6-7.5k RPM before combustion losses
efficiency, compared to around 5,000 RPM in
conventional diesels (where direct injectors simply
cant advance enough and pump in fuel quickly
enough). |
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The reduced combustion temperatures would have
the side effect of reducing NOx emissions, but the
homogenous mixture coupled with the high
expansion ratio would ensure ample time to burn
and clean combustion. Soot is mostly caused by
poor mixing of fuel and air as mentioned. |
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or just drinking the Diesel/Ethanol as an alternative to the Atkinson's Diet. |
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