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Tour Engine has Prototype II split-cycle engine running

Prototype II Tour Engine—a novel split-cycle engine—on the bench. The hot side is on the right. Click to enlarge.

Tour Engine, the developer of a novel split-cycle engine (earlier post), has its Prototype II engine running and will present details on its operation at the upcoming SAE 2012 World Congress in Detroit.

In early bench-testing of the new prototype, Tour found samples of a similar work gain value to that from a conventional control engine (129 kJ for the control and 124 kJ for the Tour engine in the example depicted below), with both operated at about half throttle and using a symmetrical Tour engine with a compression ratio similar to the expansion ratio (8:1). Other examples show similar trends, according to Tour.

However, notes Oded Tour, the Tour engine can be greatly improved by having an expansion ratio of up to 3 times larger than the compression ratio and by further differential engineering of the compressor cylinder and the combustor cylinder. Thus, the company plans in the near future to modify prototype II to have an expansion ratio of 16:1 with compression ratio of 8:1.

At first cut, roughly, we have not done any worse than the baseline engine. The way forward for the Tour engine is clear substantial improvement.

—Oded Tour

Recordings from Prototype II. Top: Five cycles showing in-cylinder pressure as a function of time. Middle: A set of readings relating to a specific cycle. Bottom left: Pressure as a function of volume for the same specific cycle (p-V): The area within the blue and red curves represents the compression work invested and the resulting combustion work extracted, respectively. Bottom Right: 1) At power piston TDC the crossover valve opens and the pressures in the two cylinders are almost equalized. 2) Timing of the spark. 3) Combustion initiation is timed to compound the maximum pressure achieved during compression (with an open crossover valve). 4) Closing of the crossover valve. 5) Power stroke is being executed in the Hot-Cylinder. Click to enlarge.

Readings from control engine. Click to enlarge.

The premise of a split-cycle engine is that segregating the intake and compression strokes in one cylinder, and the combustion and expansion strokes in another, coupled cylinder, provides a thermodynamic advantage enabling a more efficient engine. Most current split-cycle designs use a gas crossover passage or intermediate chamber to connect discrete cylinder pairs. By contrast, the Tour engine configuration directly couples the two opposing cylinders, with a single crossover valve controlling the charge flow between the two cylinders.

SolidWorks Design of Prototype II, partial transparent front view. The vertical purple part located between the two engine sides is the custom designed connecting plate that hosts the crossover valve. The hydraulic pump that is connected via a timing belt to the engine and is used to load the engine is depicted on the top. Click to enlarge.   SolidWorks Design of Prototype II, back view. The four gearwheels have the following functions: At the 9 o’clock position is the gearwheel connected to the compressor cylinder. The crossover valve mechanical cam is at 12 o’clock (since the valve actuation is both precise and fast – it opens and closes within about 45 degrees so that a large cam is required). At the 3 o’clock position is the combustion cylinder gearwheel, and at 6 o’clock is the hydraulic pump gearwheel. Click to enlarge.

The crossover valve enables the execution of an integrated cycle: the inducted working fluid is compressed and combusted as part of a single cycle, thereby avoiding piston runaway. The Prototype II Tour engine, based on two 190 cc Briggs & Stratton engines, uses a mechanically actuated crossover valve.

The Tour engine is designed to operate using conventional realistic compression ratios (8:1 to 20:1 depending upon fuel type and the use of SI or CI cycle), and is designed to fire at the end of the compression process (before any decompression occurs)—while the crossover valve remains open—very similarly to conventional engines but retaining the split-cycle thermodynamic advantages.

Firing with an open crossover valve allows the TourEngine to follow the conventional 4-stroke cycle thermodynamics, but on a split-cycle platform. (The disadvantage is a small efficiency penalty associated with a larger surface area acting as a heat sink at combustion initiation.) With complete charge transfer, the crossover valve closes; combustion continues in the hot cylinder.

The crossover valve is key to the success of the engine; it must be able to open to allow the compressed charge transfer and then immediately close (on the order of 30–50 crankshaft degrees) and transfer the charge with minimal resistance. In other words, the valve needs to be large enough in cross-section not to be a bottleneck, but also thin enough in profile to ensure minimal dead volume.

Dead volume on the compression side prevents full transfer of the compressed working fluid, while dead volume on the expansion side reduces volumetric efficiency and decreases the phase lag for a given compression ratio, which will require even faster valve actuation and therefore will be more challenging

Crossover valve and cylinder connecting plate. The engine is designed in a modular fashion such that several different connecting plates housing different crossover valves could be tested on the same engine. Click to enlarge.

The alpha prototype used a spring-loaded crossover valve. In addition to the current mechanically actuated crossover valve used in Prototype II, Tour is also developing several other crossover valve concepts including an electromagnetic crossover valve that is actuated by compression (open) and combustion (close) while the electromagnetic force is used to fine tune (hinge) the valve to close and open at the precise timing.

According to Oded Tour, Tour Engine is in discussions with several OEMs on establishing a joint development aiming on taking the concept to the next level by building an advanced Tour engine.

GM Vice President of Global Research and Development Dr. Alan Taub noted in his talk at the 2011 DEER conference that split-cycle engine technology looks promising and that GM was pushing in its R&D laboratory to see if it can get split-cycle technology “moving”. (Earlier post.)

...we may finally be entering the era where the demand for fuel efficiency will be allowing us to break away from what has become the standard architecture of our engines, and in particular the idea of separating the compression and the combustion (expansion) cylinders, into a dual stage engine. People have talked about it, a lot of people are starting to build prototypes in this, and the driving force is clear: we can see very dramatic improvements in efficiency by going to the DCDE [Discrete Compression Discrete Expansion] architecture. It takes mass, it takes cost, it takes complexity, but giving those kind of efficiency improvements, it is definitely something we need to explore further.

—Alan Taub at DEER 2011

Dr. Chris Atkinson, Professor, Mechanical and Aerospace Engineering at West Virginia University and an advisor to Tour Engine, concurs that over the last three to four years, “a remarkable openness to contemplating such new engine architectures” has emerged among the major OEMs. As for Tour Engine, he added, leaving aside the commercialization aspects:

They have done remarkably well from a technical point of view. To have a [new] engine running legitimately with comparable efficiency to a conventional engine is very much a remarkable feat. Normally, you take several steps back and then you try to work out what you’ve done wrong, whereas here, they are close to conventional already. On a shoestring budget with a minimum of people they have done remarkable things; the quality is very good—approaching OEM quality.

The Prototype II Tour Engine was built with the aid of the Israel Ministry of Energy and Water Resources.



The problem with hyper expansion is that it requires to increase the size of the engine for a given output power, on the other side increasing the compression ratio allows to increase efficiency without requiring over-sizing. So it is doubtful that this engine will do better in term of efficiency than a turbo charged downsized engine with direct injection and that would be smaller...

But Roger Pham can certainly explain this better than me


Nothing would prevent this engine from being turbocharged, even with hyperexpansion.  Gas flow working against back pressure will do work, and the exhaust is always going to be hotter and have greater volume than the intake air charge.  I doubt that this engine would have EGT as low as a diesel, and diesels still make excellent use of turbocharging.


I enjoy mechanical engineering experiments; this one is especially interesting. However, the golden age of the ICE is now fast becoming a past era as battery powered electric motors are fast becoming the future of transportation.


One thing strikes me:  delayed closure of the intake valve creates asymmetric compression/expansion with dynamic control.  This would allow a turbo to be spooled up with full engine air flow, then cutting back the intake charge to both throttle and reduce the mechanical compression back-work and put more out the crankhsaft; the turbo would continue to operate on the available exhaust energy.


One interesting possibility with split cycle engines is that they allow cooling the charge after compression so that combustion can be accomplished at much lower temperature than in conventional ICEs. This engine apparently does not take advantage of this opportunity. Likewise, it should be noted that asymmetric compression/expansion and turbocharging can be made relatively easily on a conventional ICE by using Miller system (utilizing the Atkinson cycle). This would be an aggressively downsized engine that also has high efficiency at high loads, which increase the potential for hybridization. Thus, the basis for comparison should perhaps be such an engine. Note that this kind of engine is not (yet) in production. The 1.0-liter Ford 3-cylinder engine could be seen as basis for this concept. It is aggressively downsized but it does not have the turbocharging available for employing a Miller system, implying a further development potential. Note that one feature of the Miller system is that it cools the charge to a lower level at top dead center than a conventional engine, with considerable advantage for thermal load; a known problem with aggressive downsizing. Split cycle engines could have similar advantages. I am looking forward to reading the SAE paper on this concept and, of course, compare with the Scuderi engine.

If we just sit down for the next 100 years and wait for cheap and good batteries, we might see the golden age of battery cars. Those of us who want to buy cars in the meantime are happy that the development potential of ICEs is far from being exhausted and in addition, hybridization will take us one more step forward. Recall that both energy use and CO2 emissions in the 2030 US perspective will be lower for a gasoline hybrid than a battery car (as shown by Heywood et al. at MIT).


Our atmosphere has a limited volume to be adulterated with toxic products / gases that are detrimental to organic life including us stupid humans. For those of which this process is too slow, I recommend a feedback of the exhaust into the vehicle interior to enhance the inevitable of the long run and enjoy the consequences immediately.


Why would you cool an air charge immediately after compression, then heat it by burning fuel in it?  Just burn less fuel, it's more efficient.  If you need a stoichiometric mixture, use a diluent.

Intercooling makes sense; it cuts the back-work of the next compression stage.  Cooling between compression and combustion does not.

What I'd like to see is a split-cycle engine with heat regeneration.  That would break efficiency records.


Im interrested to buy a lawnmower with this engine. Im not interrested to buy a car or an suv or a tractor-trailer truck or a train or an airplane or a ship with this engine because it is too much of a financial risk given the cost. better starting commercialisation in a small way. Do the manufacturing in china and distribute this near where i live at the local walmart. Include in the price a full refund garantee for 2 years in case of malfunctionning.


Peter XX,

'lower' is not good enough. We need to get to zero.

Roger Pham

Hi Treehugger,
This is how I see it:

1. Overexpansion a little bit (compr at 10 and exp at 13) is good for peak efficiency but the price to pay for this is lower peak output and lower efficiency at very low loads. Lower efficiency at very low loads is due to higher friction and expansion against atmospheric pressure in the case of the throttled engine with low manifold pressure. That's why Atkinson-cycle engine will need electric hybrid for power boost and for avoidance of engine use at very low loads, in which situation, the engine is completely shut off and the car runs on electric motor only.

2. Hyperexpansion with compr. at 10 and exp. at 20 or 30 (2:1 or 3:1) will worsen the above and will definitely need even a more powerful hybrid electric system with larger battery storage. The gain in peak efficiency will be paid for by much worsen part-load efficiency anywhere less than 1/2 throttle. The Tour engine design ameliorates this effect to some extend due to the fact that only the expansion cylinder needs be enlarged, while the compression cylinder is kept small, thus keeping down the engine size and friction. Careful quantitative analyses need be done to see what degree of overexpansion would be optimal for the Tour design. Hyperexpansion negates the possibility of using turbocharging or turbocompounding.

On the other hand, if one has a turbocharged diesel engine and has access to the most modern and highly-efficient turbocharger and electric turbocompounder by the use of CNC manufacturing to minimize the blade clearance to micro-meter range, one can have a more compact engine with equal peak efficiency, but with higher part-load efficiency since the turbomachinery will simply take a break at low loads and one simply has a downsized engine with low friction and high-quality combustion.


Three things here:

  1. The compression ratio of this engine can be varied by changing the phase difference between the compressor and expander crankshafts.  The peak compression ratio can be limited as desired.
  2. Differential compression vs. expansion ratios can be produced by late closure of the intake valve.  With current VVT systems, the change can be any value between the mechanical limits.
  3. To make up for lower volumetric efficiency at higher differential expansions, turbocharging can make up for the reduced air charge.  Diesel turbos regularly operate at higher output pressures than the input exhaust gas pressure; this would just harness the energy better.  Hyperexpansion doesn't preclude turbocharging; any operating point which uses a turbo waste gate or equivalent could have used hyperexpansion instead to turn the dumped energy into output.

Roger Pham

Instead of using a turbo waste gate, why not stick in a turbocompounder, electric-style, to harness these exhaust energy into electrical power. The advantage of this over an overexpansion cycle is that at part loads, the turbocompounder is detached and not contributing to engine friction nor pumping loss as would an asymmetrical cycle.


A turbocompounding system is an extra element.  This is not to say that it may not be worthwhile, but if it could be replaced by a hyper-expansion engine it raises the question of what purpose it serves.

Roger Pham

HIgher efficiency at low loads.

Roger Pham

But at higher cost for electric turbocompounding vs overexpansion engine, so Tour engine can be more advantageous if the exp:comp ratio can be optimized depending on the engine's mission, whether in an HEV vs PHEV vs non-hybrid. I would stay away from hyperexpansion on non-hybrids.


It is nice to see that you always have a different opinion than me. It is a good basis for discussion. If you have accepted the concept of intercooling, you should realize that an even colder charge is more beneficial from a thermodynamic viewpoint. I realize that I should have pointed out that this comment was not directly valid for the Tour engine; it was a general comment. Let me clarify: If you do the compression work outside the combustion cylinder, in several stages and with intercooling/aftercooling, you can get a lower temperature before combustion. Thus, you also reduce the compression work compared to a conventional engine and provide the condition for hyperexpansion and lower heat transfer losses. This is easier to accomplish on a split-cycle engine than on a conventional engine.

Current electricity production does not give us zero with BEVs – in fact greater CO2 emissions than conventional fuels – so what is your suggestion. If you suggest producing electricity in a different way than today, my answer is: please go ahead; there is plenty of coal plants to shut down before you even have to think about BEVs!



No, current electricity production does not give greater CO2 emissions than conventional fuels, where did you get that idea from?

Average fuel consumption of cars sold today is 24 mpg, resulting in well-to-wheel CO2-emissions of ~275 g/km. A 5 km/kWh electric vehicle on the US grid mix is about 120 g/km (assuming 600 g/kWh).

But then you can point to the Prius and say that it has half CO2 emissions, or ~130 g/km. Yes it is tempting to compare the EV to the most fuel efficient petrol car on the road, which represents 2% of sales, and forget about the other 98%. Another problem is that 'the EV' does not exist yet. There are less than a handful available at the moment. So any comparison is highly subjective.

Even if the announced 54.5 mpg CAFE standards for 2025 come true, then you'll have to assume that the us grid mix is still the same 13 years from now to argue that the EV is worse than a conventionally fueled car. This is highly unlikely. And even then, the petrol car wins by only a very slim margin.

On the individual level, many people link the purchase of an EV to the installation of a PV system to offset their extra electricity consumption, thereby tilting the playing field very much in favour of the EV.

The essence is this: depending on your assumptions, the EV today can be slightly worse than some petrol cars. Otoh there are ample circumstances, some under your control, some not, where it is significantly better.

Ok now, the point I was trying to make is that a petrol car can not get to 0 emissions. You can not keep increasing the mpg's forever. It will become exponentially more expensive to wring out yet more metres from a droplet of petrol. If you want to get to 0, you'll need something fundamentally different like an EV running on 100% [plug in your favourite clean technology here] electricity.

Finally, the most important argument for buying an EV now is that the technology is still in its infancy and needs to be developed before it can be mass marketed. No significant EV sales are to be expected before 2025. Sure, you can wait until 2030 or so, but you can't skip this early adopter phase and begin mass marketing out of nowhere. If you start in 2030 with the sale of EV's you're too late to 1) develop the technology and 2) ramp up mass production and 3) replace the existing fleet. That whole process takes at least thirty years.


Not to forget: many present EV-drivers are "fueling" their EVs from their own private regenerative source and commute with zero emissions.

@Engineer-Poet It is nice to see that you always have a different opinion than me.
Not always, except when I know you're wrong.  It's especially amusing when you make assertions that are so trivially wrong and the opposite is so well-documented.
If you have accepted the concept of intercooling, you should realize that an even colder charge is more beneficial from a thermodynamic viewpoint.
I'll just let someone from Nevada who teaches thermo say it:
Intercooling and reheating will always decrease the thermal efficiency unless they are accompanied by regeneration. This is because intercooling decreases the average temperature at which heat is added, and reheating increases the average temperature at which heat is rejected. Therefore... intercooling and reheating are always used in conjunction with regeneration.
Intercooling rejects heat that has done no work.  It is obviously a loss, an irreversibility, and the lower temperature of heat addition creates greater entropy.  While I can cite one example of a gas turbine that is intercooled but not regenerated (the GE LMS100), it's intended for combined-cycle use where regeneration pulls energy from the bottoming cycle.

I see Anne polished off your other claim so I don't have to.


Well, look at the MIT report that I have referred to many times on this website. That explains everything you need to know.

You should be a poet! Your comments about intercooling really show how little you know. First, you must be aware of that intercooling increases the efficiency of an engine. Any engine engineer knows that! Recall that losses in the intercooler are not the only losses in an engine. It is all about minimizing the total losses. Now, you should take as homework to find out and explain to everybody at this site why intercooling increases the efficiency of an engine. After that, you should never question my comments in the future!


Peter XX doesn't merely forego quoting the relevant portions, he foregoes linking the report at all lest he open himself to criticism on the specifics.  This is handwaving carried to extremes.

Mr. XX, I'm well aware that turbocharging and intercooling increases the volumetric efficiency of an engine.  Whether this increase the thermal efficiency depends on the average vs. peak output, among other things.

you should take as homework to find out and explain to everybody at this site why intercooling increases the efficiency of an engine. After that, you should never question my comments in the future!
On the contrary:  you should put forth the thermodynamic basis of all your assertions, because too many of them have been bogus in the past and you have a credibility gap that cannot be made up by handwaving.


Now you are in deep trouble. It is a poet against all engineers. You simply did not know that intercooling improves efficiency. This is a well-known fact among engineers. I have noted that you pretend that you are an expert in this field but this time, you went too far and exposed you lack of knowledge.

Ralph Miller, more famous for the Miller system (or Miller cycle), proposed using intercooling on large gas engines long before his other mentioned invention. He was ridiculed by many at that time, with claims that you cannot improve a heat engine by taking away heat. Guess who was right? Ralph Miller, of course!

I have stopped giving free lectures on this site. You are one of the reasons why. However, with all the documented knowledge about the positive effects of intercooling on efficiency, I do not have to explain anything this time.



Link the exact study that is the basis of your claim. Heywood has done more than 1.

I have shown you that the average car now emits twice as much as an EV. There's no way around that. And the recent Heywood studies I assumed might be the ones you're referring to do not challenge my figures in any way. And I am not interested in cherrypicked models (Prius-only scenario's). It's what people buy that counts, not what is available.


Ooooh, Peter XX says I'm in trouble.  I'm quaking in my boots shoes slippers.

We know what we're going to find if he ever links to this study:  what it says is true, but inapplicable to the specific claim he made.  Rather than admit his error, he'll bluster even more.  It's the frustration of the credentialed fool at being called out.


Oh, wait, I think I remember when he did link to that study.  Conveniently for him, it's behind a paywall.  That's how he can assert that it supports him; nobody is going to pony up forty bucks or so just to get the goods to prove him wrong.

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