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Concept: Zajac Motors Proposes Split-Cycle Engine with Separate Combustion Chamber; 15% Improvement on Diesel Fuel Economy

Basic elements of the Zajac engine. Click to enlarge.

Zajac Motors is developing a split-cycle engine concept that significantly prolongs fuel burn time through the use of a combustion chamber external to the compression and expansion piston chambers. John Zajac, who presented a poster session on his concept at the Diesel Engine-Efficiency and Emissions Research (DEER) conference this week in Dearborn, Michigan, claims that his engine will deliver 15% better fuel economy than a diesel, run on any liquid fuel, and be essentially non-polluting.

The Zajac engine uses new head parts that are compatible with a standard diesel engine block. The in-block cylinders are split into compression and expansion roles, and connected by an external “hot wall” combustion chamber. The engine presumes the use of new rotary valves, also developed by Zajac. The combustion chamber, valves, and control systems are the core of Zajac’s IP.

By way of comparison, the Scuderi split-cycle engine also divides an engine’s cylinders into discrete compression and expansion roles, but transfers the intake air compressed in the compression cylinder via a high-pressure gas passage to the power cylinder for combustion. (Earlier post.)

The Zajac compression cylinder takes in air and compresses it, with a stroke 10% longer than that of a conventional diesel engine. The compressed air charge is input into the combustion cylinder where it mixes with the fuel charge, ignites and burns. The combustion chamber’s size, shape and insulating materials create a “hot-walled” environment where fuel, air, and pressure can be maintained at a constant temperature. The size of this energy storage chamber also extends the length of time that gas is held resident in the combustion chamber—10 to 100 times longer than in a traditional engine.

This results in a complete and clean burn. The complete combustion delivers the improvement in fuel efficiency, and also virtually eliminates the production of CO, unburned hydrocarbons and soot. NOx is prevented by keeping the combustion temperature below the 1700 K formation threshold, Zajac said.

In the expansion (power and exhaust) cylinder, the power stroke is up to 220% longer than that of a conventional engines, allowing full expansion. The Zajac engine exhausts gas at 1.5 psi above atmospheric pressure, reducing waste and minimizing noise.

Zajac has 19 patents pending on his engine and is looking for development partners. Zajac Motors is an IP company, he said, and doesn’t intend to manufacture the engine.



One more ICE concept in the air...intersting though rotating valves are new at all but have proven to be difficult to make work properly (leak under high pressure, friction, lubrication problems. Also an external combustion chamber will significantly increases thermal losses, but the idea is good

Let us see a working model tested by an independent lab.

A low compression, super low pressure engine does not sound more powerful or better.

I like this one better:

facing offset dual piston with conjoined combustion chamber.


No throttling or heat transfer losses there.Using 2 pistons to do the work of 1, genius.

Apparently theyve never seen an IDI diesel before.


I'm no engineer, but isn't high temperature one of the the keys to thermal efficiency? Isn't the cylinder cooling enabled by alternating between cool intake charge and hot combustion one of the reasons internal combustion engines can be made inexpensively without exotic alloys? Also, what is the specific power of this thing--looks intrinsically heavy to me.

I'll be interested to see a running prototype with some real-world test results.


Nothing new. That's the ikV-Verfahren (internal continous combustion process) developed by Dr. U. Rohs at the RWTH Aachen in 1970. With his company GIF (Gesellschaft für Industrieforschung mbH) he developed an engine based on this process (ikV-Axialkolbenmotor), build only with common engine parts, which they presented last year. He claimed an efficiency of more than 60%.

The obvious problem (as pointed out by Nick already) is the thermal stress.


Only if it had a few extra valves Joe. ;)


"He claimed an efficiency of more than 60%."

Cool! We have our range extender....

Franklin E. Fraitus

I designed an engine similar to this years ago. However, the idea was to use firewood as the energy source. (likely pellets or chunks). With many of the same ideas Zajak incorporates. The interesting part is, this is quite similar to the turbine engine and could likely use much of the same technology.

Some here might find this site of a wood burning turbine engine interesting: Video at bottom of page.

So, Of course I like this idea! One point everyone seems to miss with such an "experiment" is that the power stroke can be longer than the compression stroke. Which is, of course, the holy grail of reciprocating engine efficiency improvements.

why not just use a compressor (suppercharger, turbo, etc) instead of a piston for the compression part of the equation? would seem simpler.

The idea of an external combustion chamber is interesting in that the chamber itself can be made of more exotic/appropriate materials like ceramics. You dont have the problem of a seal and friction/wear to worry about compared to a ceramic cylinder wall.


It doesn't matter. All combustion engines are evil. This is simply extending the use of evil devices. You must all walk to and from your destinations. Go Obama! Change! Hope!

Roger Pham

<<"why not just use a compressor (suppercharger, turbo, etc) instead of a piston for the compression part of the equation? would seem simpler."

Rotary compressors suffer from leakage around the edges and would not be practical for a pressure ratio of 20-40 as would be required for a reasonably compact piston engine. The exception is very large gas turbines having pressure ratios of 30-40, wherein the rate of leakage is relatively small due to the sheer size of the turbines. For a compression ratio of 13:1 and with polytropic compression, a pressure ratio of ~25 would be generated. Because of the high pressures involved and the temperature of 1700K, the combustion chamber must be kept as small as possible, or structural problem and heat loss will be significant. Gas turbines use uncombusted air circulation around the combustion chamber for cooling, but in so doing, resulting in higher flow rate and lower turbine inlet temperature of about 1200K, hence lower Carnot efficiency and lower specific power output. Ceramic coated combustion chamber has been used, but can be expensive.

Franklin E. Fraitus

(failed to give his name) posted: """It doesn't matter. All combustion engines are evil. This is simply extending the use of evil devices. You must all walk to and from your destinations. Go Obama! Change! Hope!""

Wow, combustion engines evil? Hard to believe, but maybe so. I suppose the fires from hell power them. However, it seems to me that the simple use of power has resulted in major human advancements. Including, but not limited to, extended lifespan, life saving drugs, Rescue teams that can travel large distances in short times and of course a higher standard of living.

I think it better to refine and cure the ills of our energy technology than to scrap it. Not sure that Obama has any real scientific experience in this area. So, you can hope that he does, but I think not.


The anonymous one was clearly sarcastic.

Combustion engines have three major problems:

  1. They require chemical fuels, preferably liquids (for density and ease of transfer).
  2. They are quite inefficient compared to e.g. batteries, converting perhaps 40% of the fuel energy to work (and far less on average).
  3. They pollute the air with both chemicals and noise.
15% is a non-trivial improvement over diesel, but isn't going to bring the combustion engine into electric-propulsion territory.


roughly half the power for a given engine weight - 2 cylinders to do the work of one, plus additional combustion chamber weight.


woops, sorry, that's probably wrong. I guess each cylinder has only two strokes per cycle. Yeah, thermal stress seems a big issue. Throttling and even heat losses could probably be fairly well controlled.

Roger Pham

No, joeblow, you're not wrong.
Twice the weight for the same power in comparison to a diesel. The hot cylinders can only develop so much power, limited by heat removal. Same heat limitation in a diesel 4-stroker, but, you have twice as many power-producing cylinders for the same weight, therefore, twice the power for a given number of total cylinders.



you are wrong. the ICE is the most efficent engery coversion device in the world (~40-50% for a diesel engine compared with an electric plant in the 30%). Comparing a 2nd law efficiency (batteries) with a first law efficiency (fuel comversion) is wrong and misleading.

PS Making electricity and batteries prodeces pollution too.


Quoth joe:

the ICE is the most efficent engery coversion device in the world (~40-50% for a diesel engine compared with an electric plant in the 30%).
High-speed diesel:  35-40% peak
Phosphoric acid fuel cell:  35%
Low-speed diesel:  ~50% peak
Combined-cycle gas turbine:  60%+
Molten-carbonate or solid-oxide fuel cell:  60%
Direct-carbon fuel cell:  70-80%

The combustion engine requires a chemical fuel.  Whether produced as inefficiently as rapeseed-oil biodiesel or as efficiently as sugar cane ethanol, the watts delivered per square meter is a tiny fraction of what a PV panel and battery achieve.

PS Making electricity and batteries prodeces pollution too.
But this pollution is rarely produced at street level inside our most heavily populated areas.

Roger Pham

Optimized H2-ICE can achieve 50% efficiency, and almost zero pollution. If the H2 is made from PV or wind electricity, then overall efficiency will not be too far behind BEV's also charged from renewable electricity.


For large values of "not too far behind", perhaps.

Henry Gibson

It the US and other places, there are religions that believe that engines are evil. They use pressure lanterns with the false assumption that they are less evil than electric lights because they were invented first and are lower tecchnology.

The absolute efficiency of the operation of biofuels is very low compared to even the worst solar cells on the market.

No automobile is designed with efficiency as the total goal. Almost no automobile is bought with efficiency as the main goal. Over all economy of operation is even less considered.

COATES LTD actually sells engines with rotary valves. RCV sells small engines with rotary valves. It is a shame that the fact that this engine can operate in air hybrid mode is not mentioned. Compressed air can be used to start and run this engine.

In small sizes, pistons are the most efficient compressors, but turbines can be relatively efficient as expanders in small sizes as turbochargers demonstrate. Some large piston aircraft engines had exhaust turbines coupled through gears to the crank shaft. Some free piston engines got all of their power from turbines operating from the exhaust gas.

An ideal small engine would have a piston compressor, high pressure air storage tanks, ceramic combustion chambers with graphite fiber reinforcement if necessary and nozzels that spin a turbine for power take off and to operate the piston compressor.

Capstone microturbines have been used in hybrid buses where the lower theoretical efficiency compared to a piston engine is compensated by the low operational efficiency of engines in buses. Capstone turbines need no coolant or lubrication systems. Smaller much higher speed but lower power turbines could be used in hybrid automobiles. Such turbines can burn any fuel like the proposed engine.

I can now stop thinking what advantages the proposed engine would have as it has been on my mind for twenty years at least. ..HG..

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