Caterpillar engineers have been exploring a novel 6-stroke compression ignition engine cycle in search of a low-emission system that retains fuel efficiency. In a paper presented at the SAE 2014 World Congress, they reported on their investigations of the 6-stroke cycle for near-stoichiometric and lean operation.
The primary difference between the conventional 4-stroke and Caterpillar 6-stroke cycle is the addition of a second compression and combustion stroke immediately following the primary combustion event. The fourth stroke omits the conventional exhaust event in order to recompress the combustion gases. This approach differs from the traditional 6-stroke approach in which water is injected into the cylinder after recompression to extract additional expansion work from the hot combustion products, the team noted.
The purpose of the second combustion event is to utilize the excess oxygen in the cylinder by providing an extended period of time in a high temperature environment to promote oxidation of soot particles formed during the first combustion event.
The first thing that should be appreciated when considering the 6-stroke cycle is the number of added degrees of freedom that are introduced by the inclusion of a second combustion event. First and foremost, the quantity of fuel that is injected during each of the combustion events can significantly impact the combustion performance.
…It should be noted that these injection events are not mutually exclusive. Because much (or all) of the combustion products from the first combustion event are retained in the cylinder, the temperature, pressure and oxygen concentration at the beginning of the second combustion stroke can be impact by the fuel split. In general, a larger fuel split will consume more oxygen and result in a higher bulk gas temperature and, subsequently, higher pressure on the second stroke. The converse is also generally true; burning less fuel during the first combustion stroke will give lower temperature and pressure for the second stroke.
…The addition of a second combustion event also doubles the flexibility of other standard combustion parameters such as injection timing and number of injections. Even the injection pressure could be independently varied, though with greater difficulty.—Williams et al.
The 4-stroke cycle consists of one power stroke for every two crankshaft rotations; the 6-stroke cycle consistes of two power strokes for every three crankshaft rotations—i.e., seemingly a higher power density. However, due to a number of factors, the Caterpillar team pointed out, the 6-stroke engine should be able to deliver a power density that is on the same order of, but not likely substantially better than, the 4-stroke engine.
The researchers used a single-cylinder engine adapted from the Caterpillar C-15 (15-liter) diesel. They made several modifications to switch between 4-stroke and 6-stroke cycles, the most significant of which were the actuation of the intake and exhaust valves. They team used the Lotus Active Valve Train (AVT) fully-flexible valve actuation system in the testing.
AVT uses hydraulic pressure provided by a separate pump and power source to actuate the engine valves, thereby decoupling the valve actuation from the engine rotation, and enabling flexibility in vale timing and switchability from 4-stroke to 6-stroke without hardware changes.
Another major change was the adaptation of the Engine Control Module (ECM) to recognize a cycle with three crankshaft rotations rather than two as well as the use of a partial exhaust event known as Intercycle Blowdown (ICB) used to reduce temperature and pressure before the start of the second compression stroke to keep the 6-stroke engine from reaching engine design limits at a lower load factor than an equivalent 4-stroke.
Among the main findings of the study were:
Stoichiometric 6-stroke operation at low load showed potential for low-soot operation. At higher loads with ICB, however, the fundamental temperature and pressure constraints of the engine presented insurmountable challenges to the gas exchange process, which severely limited the overall efficiency of stoichiometric 6-stroke operation.
10x reductions in engine-out soot were seen where small quantities of fuel were used on the second combustion stroke, albeit at a sacrifice of efficiency. Beyond a fuel split of 70%, the production of soot on the second combustion stroke exceeded any oxidation benefit.
According to the simulations, there was potential to improve efficiency by controlling heat release phasing and burn rate of the second combustion event. This was not shown on the engine due to limitations of the combustion chamber.
The 6-stroke exhibited lower combustion irreversibility, lower exhaust energy and port flow losses, but higher heat transfer and friction losses. The result was that the 6-stroke cycle exhibits slightly higher indicated, brake and 2nd law efficiences, all else being equal.
10.4271/2014-01-1246 Citation: Williams, D., Koci, C., and Fiveland, S. (2014) “Compression Ignition 6-Stroke Cycle Investigations,” SAE Int. J. Engines 7(2) doi: 10.4271/2014-01-1246