The Stratified Charge Revolution That Could've Been

[kernals12]

Imagine an more efficient than a diesel, cleaner than any normal gasoline engine, could run on just about any fuel, and merely needed an oddly shaped cylinder head to work.

These were the promises of the stratified charge engine.

First a background: a normal car engine generally runs at a stochiometric air fuel ratio of 14.7:1. Anything lower than that is considered "rich" and anything higher is considered "lean". I'm not a mechanical engineer, though I have read a lot of articles about this, but my understanding is that an engine that is too rich will waste a lot of fuel and create a lot of smoke. An engine that is too lean will fail to work at all and create lots of nitrous oxide emissions, although after a point the NOX levels taper off.

But if there was some way to make an engine work at very lean mixtures, you'd have an extremely efficient engine, as more energy is absorbed by the air in the combustion chamber rather than being lost to the cylinder walls. Furthermore, you don't need high octane fuel, as the excess air will keep knock under control. And you can do without a throttle, simply varying the amount of fuel injected, just as diesel engines do.

One way to do that is to "stratify" the air fuel mixture. The overall mixture would be lean, but near the spark plug would be a rich mixture that easily ignites and in so doing creates a flame jet that ignites the whole mixture.

The idea of stratified charge dates back to the early days of the automobile. The pioneering British engineer Henry Ricardo patented such an engine in 1918. He used a separate prechamber that would be given a rich mixture to ignite a lean mixture in the main combustion chamber. The idea was held back by cost and combustion instability.


The idea would be picked up starting in 1949 by the Texaco oil company. Why would an oil company develop an engine that would cause them to sell less fuel? It's because they were annoyed at the cost of refining ever higher octane rating gasolines to meet the auto industry's demand for higher compression ratios. If this worked, then they'd be able to get a lot more motor fuel out of each barrel of oil they processed



The Texaco process was different from Ricardo's. There was no prechamber, instead fuel was injected directly into the combustion chamber into either a bowl shaped piston or a hemispherical chamber head. The fuel would be ignited by a spark plug and create a swirling trail of fire that would ignite a super-lean mixture.

Interest in the stratified charge engine began ramping up quickly in the second half of the 1960s as crackdowns began on tailpipe emissions.

Among Detroit automakers, Ford showed the most interest, developing what they called PROCO (programmed combustion). Just like Texaco, they used a bowl-shaped piston, but they chose to forgo the swirl ignition in favor of using 2 spark plugs (earlier prototypes used on spark plug with an elongated electrode). There were problems though: the high combustion temperatures led to high nitrous oxide emissions. To solve this, they used exhaust gas recirculation. Due to the slower combustion speeds, this hurt fuel economy.



But one company did succeed: Honda, which at the time was mostly a motorcycle company in the eyes of the public. They carbon copied Ricardo's idea of a valved prechamber. They called it "Compound Vortex Controlled Combustion" or CVCC.

GM CEO Richard Gerstenberg scoffed at the system, saying 

"Well, I have looked at this design, and while it might work on some little toy motorcycle engine...I see no potential for it on one of our GM car engines."

Sochiro Honda was insulted and to prove them wrong, he bought a 1973 Chevrolet Impala with a 350 cubic inch V8, had it shipped to Japan where his engineers fitted it with CVCC heads, then sent it back to the US where it was tested and found to be able to meet the 1975 emissions standards without a catalytic converter.

But all was not well. Contrary to urban legend, the tests found the CVCC Impala was much less fuel efficient than stock (due to slow combustion) and it had higher NOX emissions.

And nitrous oxide emissions would be a dealbreaker. In 1981, 3-way catalytic converters arrived. They worked by having hydrocarbons strip off the oxygen from NOx. In order to have HC emissions high enough to make it work, engines had to run at stoichiometric air-fuel ratios.

Clearly what would be needed to make this work is an ignition method that can ignite a lean fuel air mixture with a high enough portion of exhaust gas recirculation to keep the temperature low enough to prevent NOX formation. Plasma jets and lasers have been investigated but issues of cost and reliability scuttled those paths. And now that electric cars are arriving, it's unlikely we'll ever see those hyper-efficient, smog-free, omnivorous engines, which is kind of sad.

[abefroman329]

And now that electric cars are arriving, it's unlikely we'll ever see those hyper-efficient, smog-free, omnivorous engines, which is kind of sad.

We would if there was a way for them to run using fuel from renewable resources, though.