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Concept: A Universal Engine Offering Flex Fuel and Flex Combustion

A cutaway illustration of a basic AVFIC engine cylinder unit

Two California inventors have designed and patented the ultimate flex-fuel and flex-combustion engine: an engine that can adapt in real-time to a variety of petroleum-, bio- or gaseous-fuels using the appropriate combustion mode, including spark-ignition, compression-ignition or HCCI variants.

One of the key technology enablers behind the Adaptive Variable Fuel Internal Combustion engine (AVFIC) engine for inventors Walt Froloff and Ken Miller is the digital valve.

With the digital valve, the engine can be totally electronically controlled. The cam was the last mechanical constraint.

It’s like when the first calculators came out, the manufacturers were giving the functions that a slide rule could do, just that you could now have a little screen with buttons. That was novel and neat, but it didn’t exploit the real power of the calculator, which was to become programmable and much more useful than just pumping out a few more decimals of accuracy.

[...] I got to thinking about how we would go about designing this engine with the digital valve and realized that we were not subject to the same constraints that thousands of engine designers suffered under.

We were not prisoners of the 4-stroke or the 2-stroke. We could program any sequence. Furthermore, we could program parts of strokes.

[...] The industry has a mechanical device—the engine— which has been real useful as a slide rule, by analogy. But we’ve invented a programmable engine, a calculator by analogy, and there is no limit to what this can lead to.

—Walt Froloff (email correspondence)

The AVFIC engine has no camshaft; cylinder valves, other cylinder components and stroke sequences are electronically and individually controlled.

This leads to the theoretical ability to control fuel consumption, ignition timing, air intake, exhaust outflow and variable effective stroke length to manage fuel combustion in each cylinder in situ and in real time.

To realize that that potential requires another key enabling technology: suites of tests for various ignition modes that can be made on the fuel in use to determine what’s going on in the cylinder. Only with such information about combustion characteristics can the AVFIC engine reconfigure itself to run optimally to produce the highest power under a given set of constraints.

Essentially, the engine is controlled to act as a laboratory with series and suites of tests programmably controlled to determine the least engine-damaging ignition method to use by the engine that will yield the highest crankshaft power output for a particular fuel.

Froloff and Miller were influenced in their approach to devising in-cylinder testing by a recent work done on a method called “pressure-ratio management” (PRM) which can be used in computer-based, closed-loop, engine-combustion control to better manage air-fuel ratio, including the fuel balance between cylinders, ignition timing and EGR dilution.

Rather than using a direct probe that must be exposed to the combustion gases in the cylinder (see this post on Nissan’s and Stanford’s work in that area), PRM relies on a cylinder pressure sensor. From the pressure signals transmitted by the sensor, the PRM system calculates the fraction of the fuel burned in a cylinder.

By providing such a degree of granular control of each cylinder’s combustion, the AVFIC engine opens up a new range of possibilities.

Inside, we could not only compress air, we could create a vacuum, we could just suck air for braking and we could also use compressed air to push pistons.

The industry is still just thinking about rerouting exhaust, when you can reroute any of the stroke outputs. You can run rich or lean for a few strokes to accomplish an end and then go to another stroke or sequence.

Froloff and Miller have set up a company—InspirEngine LLC—and are looking for funding to further develop this. The duo have proposals in to Sturman (the digital valve company) and GM, and are talking with UC Berkeley.

With AVFIC as a foundation, they have gone on to propose a Dynamically Reconfigurable Internal Combustion Engine (DRICE)–air hybrid. (More on this in an upcoming post.)




All I can say If (a big IF) this technology lives up to its promises, it would be a huge step toward automotive energy independence.


Yeah, this is pretty cool.

I remember a few years ago I read that detroit diesel (or was it caterpillar?) had some prototype engines with solenoid driven valves.


A couple of years ago, when Renault came back to Formula 1 they used a V10 with an angle of 110º, and solenoid driven valves.

But they didn't use it for many time.

I guess that you also have to change the compression ratio, if you want to use different fuels. Saab had a great engine concept that could do that:


Brazil is already outselling flex cars than normal cars. They can work with any mixture of ethanol and gas. VW and other companies have started catering to that market.


I love the idea of using a compressed air hybrid drive system. It greatly improves the efficiency of regenerative braking. Batteries cannot be recharged as fast as they can be discharged. Also the voltage out of a generator is a product of speed x field current so as an electric car slows its ability to recharge drops. The pressure output of a compressor is not dependent on speed.

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