MUSIC engine of the future

[Brian Ellis]

I'm not one to use the term 'breakthrough' lightly, but I think it may be applicable to the Merritt Unthrottled Spark Ignition Combustion engine, after studying its theory. Read this link for a first contact.

Let me give a little background. A conventional petrol/gasoline engine, unlike a diesel, uses a ± constant fuel/air ratio at all speeds, to ensure optimal ignition. To achieve this, at low speeds, the air intake is throttled back. Nevertheless, the combustion within the cylinder takes place at a constant temperature. This is why the efficiency is comparatively low, compared with a diesel. The latter has no throttle, so that a full dose of air is taken into the cylinder at each cycle. On the compression stroke, the adiabatic compression of this full dose of air causes its temperature to rise. The fuel is injected in, in the form of a fine liquid spray, and each microdroplet of fuel combusts because the air temperature exceeds the flame point of the fuel. At low engine speeds, little fuel is injected, even though the volume of air is great. This technique cannot normally be applied to a petrol engine with spark ignition, because the petrol:air ratio must be near-stoichiometric for ignition of the fuel.

Prof. Merritt had the idea of stratifying the air in the cylinder, so that the petrol is injected away from the spark plug, and a swirl of the laminar stratified air/fuel mixture, in near-stoichiometric proportions is taken to the plug, where it is ignited. This is with unthrottled air, where a full dose is taken into the cylinder at each cycle. In practice, this is done in a very small combustion chamber, housed in the cylinder head. A second injector injects more fuel, according to power requirements, past the mini-combustion chamber into the main combustion chamber and this burns as soon as soon as it finds enough oxygen, being ignited from the first injection. Obviously, the timing of the two injections and the spark are critical. The fact that the air is not all used at low speeds lowers the temperature of the combustion and increase the efficiency.

This link gives diagrams and a better description while this link shows the practical implementation.

I think this is an exciting development because the extra cost for a 20% efficiency improvement would be very small for mass manufacture. I'd love to see it applied to an Atkinson cycle engine: a good 50% efficiency???

[Dr Mordrid]

Yet another version of a stratified charge engine using a pre-chamber, which IRRC Benz had in diesels around 1924. Honda, Mazda, Mitsubishi, Volvo, Jaguar and others have been developing/making them for gas engines for years.

Honda CVCC



MUSI

[Brian Ellis]

AFAIK, the prechamber is not new nor is it claimed as such. What is claimed is the engine has no throttle and the injected fuel is not necessarily stoichiometric, because of the fact that the "economy" injection takes place at the opposite end of the prechamber to the plug, and the "power" injection is totally separate. Whatever, this is a lab-tested technology that works and appears to work well on real engines. This is something that could be implemented in a very short timescale to full scale road tests. A 20% (proven) or 25% (anticipated) efficiency improvement is not to be sneezed at for a relatively minor engine modification. The improvements from stratification, up to now, have never been convincing because the changes of flow rate due to throttling are a necessary cause of compromise.

[Evildead666]

BMW engines use VANOS on the inlet and oulet valves, allowing the valve height to be varied, infinitely, and at any moment.

I think we are thinking too complex, we need to find a simple solution...

Looking at a Wankel engine, there are suprisingly little moving parts, and i'm sure that there is the tech out there to make the problem of wear and tear go away...
There may be a lack of torque, but thats what Turbo's are for

[Dr Mordrid]

There may be a lack of torque, but thats what Turbo's are for

Turbo's big gain is at higher RPM's. Before that the turbo isn't operating at its peak efficiency. Small turbos are better than large ones at low RPM's of course, but still. If you want boost that'll occur at low RPM's, almost from idle, you need a supercharger.

[cjolley]

I wonder if using a tubo instead of crankcase pressure to charge a two stroke could be combined with the stratified charge/no throttle to create a piston engine with virtually no energy lost to pumping.


PS There is also the twin turbo: two small turbos, one for lower rpm and a second comes on when increased airflow is needed.

[MultimediaMan]

Two Stoke engines, whether super or turbocharged, spark or compression ignited, can be very very efficient engines, which don't always have a throttle body (Look at a M.A.N. Diesel: no throttle body, and no carbureator with >50% Thermal efficiency for the largest models), but only within a very small range of RPM (like 10RPM for one of those beasts) ... Preferably a constant RPM Engine... otherwise, some of the intake charge will be lost out the exhaust (Too slow of an RPM), or it will not get a complete enough intake charge for good combustion (Too high of an RPM).

As far as these "throttle-less engines" work, it predicated on a very delicate balance of temperature, airflow, fuel delivery and load on the system to work. I am not att certain this will progress beyond a lab curiosity: the margins this engine runs in are typically found only in research engines (the kind with Quartz cylinders and square pistons). I would think that Flame Front propagation would become the limiting factor to compression and RPM in these "long ignition systems" which use a preignition chamber. The Pre-Ignition chamber has a "tuned RPM" which is separate from the rest of the system; it will work more efficiently within a certain RPM range. It could be that the designers chose to go with this technique on conventional engines to smooth peaks and valleys which may exist outside of the pre-ignition chamber.

Looked at singularly, it is ugly as hell; but when it all works together, it is near perfect. I am reminded of an engineer for Rolls Royce who oversaw the Pegasus engine development: under him were several teams who engineered their part of the engine to be "perfect" on the test stand, but when put together, the assembled engine didn't run well at all - well below target figures. With much cajoling and compromise, he got the teams working together, and by each one sacrificing 1% to 5% on their individual raw performance numbers, they were able to increase the total output of the engine to more than 5% above the "theoretical maximum".

I think it is important to remind everyone that Internal combustion engines of all types are nothing more than air pumps, with a little chemistry mixed in. The more air the engine can move, per unit of fuel consumed, while still staying in one piece, minus the losses thermal or mechanical is all of it, in a nutshell.

The Rest is either tinkering with thermal or mechanical efficiency, which has been the game we have played with ICEs for the last 100 years.

It is far easier to make an air pump very efficient mechanically and volumetrically at a fixed RPM than at a range of RPMs. Why we are still messing with variable RPM engines, for which there will never really be a "solution", is becoming more of a mystery to me everyday. Trains use them, ships use them, generating stations use them, and they work very well. Why hasn't the automobile industry looked at them more seriously?