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New ACE researchers propose new diesel combustion concept; pathway to >50% BTE without WHR

A team at Japan’s New ACE Institute—an industry-funded research initiative founded to develop a new diesel combustion concept—has developed a new diffusion-combustion-based concept with multiple fuel injectors to overcome the trade-offs of thermal efficiency with energy loss and exhaust emissions typical of conventional diesel engines.

In a presentation at the 2016 SAE High Efficiency IC Engine Symposium, Noboru Uchida, general manager of research for New ACE, outlined the basic approach, which he said potentially offers a pathway to greater than 50% brake thermal efficiency without the use of waste heat recovery systems. A paper on their concept is also published in SAE International Journal of Engines.

The objective, said Uchida, was to find the optimum combustion strategy which overcomes the complex trade offs among emissions, brake thermal efficiency (BTE) and energy losses based on conventional diesel combustion with more degrees of freedom.

The proposed engine employs neither low temperature combustion nor homogeneous charge compression ignition combustion; it implements a kind of Sabathé cycle (limited pressure dual cycle in which heat addition occurs partly at constant volume and partly at constant pressure).

Uchida noted that the Sabathé cycle is superior in suppressing the in-cylinder pressure increase rate and in reducing losses by constant pressure combustion. The New ACE concept is to enable the consecutive heat release of premixed and shaped diffusion combustion areas within combustion chamber; the premixed combustion area and the diffusion combustion area are temporarily and spatially isolated.

Piston crown design, injector orientation and spray directions. Okamoto and Uchida (2016). Click to enlarge.

To achieve this, the cylinders are fitted with three injectors: one vertically mounted at the cylinder center as in a conventional direct injection diesel engine, and two additional injectors slant-mounted at the piston cavity circumference.

The sprays from the side injectors are directed along the swirl direction to prevent both spray interference and spray impingement on the cavity wall, while improving air utilization near the center of the cavity.

Fuel sprays from center and side injector might interfere with each other if fuel is injected at the same time or center injection timing is later than side injection timing. The team shaped the piston cavity to achieve a relatively large bore but deep bottom to avoid wall impingement by the side injectors, even at high compression ratios.

The desired heat release rate is obtained by independent control of injection timing and duration (fuel injection pressure was kept in constant) for each fuel injector. The center injector fires first, followed by the side injectors.

Experimental results showed reduced friction loss, heat loss and NOx emissions, while maintaining indicated thermal efficiency by suppressing the peak cylinder pressure, bulk average temperature, and spray flame impingement to the cavity wall.

Additionally, the New ACE researchers achieved a simultaneous reduction in smoke and NOx emissions was achieved without any deterioration in CO (carbon monoxide) and THC (total hydrocarbon) emissions, even compared with conventional diesel combustion.


  • Okamoto, T. and Uchida, N. (2016) “New Concept for Overcoming the Trade-Off between Thermal Efficiency, Each Loss and Exhaust Emissions in a Heavy Duty Diesel Engine,” SAE Int. J. Engines 9(2) doi: 10.4271/2016-01-0729


Thomas Pedersen

Certainly interesting results. However, it remains to be seen whether the resulting fuel savings can ever amortize the cost of 3x fuel injectors that already contribute a significant fraction of the high cost of a diesel engine. Perhaps this strategy allows the injection pressure to be lowered just enough to use cheaper injectors.

Now I'm waiting for all the "why don't they just spend all the money on battery research" comments...

Not that I have anything against neither battery research, nor BEVs, it's just tiring to keep reading.


You can pay for the 3 injectors with an OPOC on a single cylinder--that's 2 or 4 cylinders on one to two borings. Or maybe just use a two-stroke piston, with some heat regeneration at the exhaust (which would go a long way with the so called "constant volume" step of the heating.

With the cost of emissions control and electronics, I'm not sure injectors are that costly.

Brian Petersen

OPOC will not allow that injector and combustion chamber geometry. (Not saying it wouldn't allow some other geometry - it just won't allow that one.) That engine design is a whole new set of issues to deal with.

The injectors would no doubt all be fed by the same common rail fuel supply. Pump and pressure regulator stays the same, fuel filter stays the same. It adds more injectors and more wiring to them, that's the added cost. The exhaust aftertreatment is the expensive bit. I have my doubts that this system could improve combustion efficiency by enough to remove the need for DPF and NOx aftertreatment.

A system like this would likely have its cost/benefit analysis work out better for a large engine, such as heavy truck applications, as opposed to automotive.


How would OPOC disallow the injector geometry? My understanding is that injection through holes in the cylinder walls is now possible with new mettalurgy. That's a few degrees of freedom to advance the ACE concept. Apparently the key is not to activate the injectors at exactly the same time. But maybe for the effort some preheating within the injectors themselves will be the ultimate key to controlling mixing and viscosity.

Brian Petersen

The OPOC design will not allow this injector geometry nor will it allow a conventional injector geometry with the injector in the cylinder head pointed straight at the center of the cylinder, because it has no cylinder head (pistons face each other). All it means is that this research was never intended to be applied to an OPOC and to make an inference that it somehow could be, is not valid.

This research was meant to be applied to a normal piston layout with a fixed cylinder head and 4 valves per cylinder with all three injectors fitted into real estate in the cylinder head that doesn't contain valves.

OPOC requires side mounted injectors and flow patterns within the combustion chamber that are nothing like those of a normal bowl-in-piston with central injector design.


"why don't they just spend all the money on battery research"

The near-future is not pure electric, nor pure thermal.

It's plug-in hybrids (Small batteries for the first 50 miles, and ICE for longer distances).
It will be the cheaper and more efficient solution.

And every contribution to improve the IC engine is in the good direction.

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