FOUR-STROKE ENGINES
DIFFERENT STROKES: ENTER THE COMBUSTION ALTERNATIVES We’re all familiar with diesel cycle and spark ignition engines, but there are a couple of new kids on the block, writes Stevie Knight. Surprisingly, they might fit right in... The idea of Homogeneous Charge Compression Ignition (HCCI) has been kicking around for a while, says Maxime Pochet of ECAM Brussels, adding that in many ways “it’s not that different to other four strokes”. However, while the diesel cycle squeezes air, then sprays in fuel to trigger ignition and SI varieties set off a mix with a spark, both approaches have their limitations. Spark ignition engines have to stay below knocking conditions, losing out on higher efficiency. Diesel combustion leaves pockets of unmixed fuel and air: soot results from a lack of oxygen to complete the burn and NOx arises from fusing spare oxygen and nitrogen at high temperatures - also an issue for SI engines. Instead, HCCI mixes the fuel and air in advance, squeezing it so it ignites in one go, rather than spreading along an advancing flame front. The lower temperature, more uniform combustion results in lessened heat loss and potentially substantial fuel savings. Its high thermal reaches over 50% on the test bench that’s more than 40% better than its SI counterpart. But most importantly for the marine industry, the lack of rich-mixture hot spots results in negligible soot while NOx is limited by the lean mixture, requiring little or no after treatment; catalytic converters mopping up the slightly raised hydrocarbon and CO emissions. In fact, it’s not that complex a transition. Intriguingly, it’s possible to switch diesel engines over to HCCI operation without a lot of trouble, says Pochet. He explains that creating a unit for his testbed “is just a matter of taking a normal diesel engine, adding a pressure sensor, an injection point, filling in the piston bowl and machining the piston head to provide the required compression ratio while minimising the heat loss exchange area”. But HCCI can also be utilised for alternative fuels. There’s been work on many fuels, such as ammonia, ethanol and hydrogen, but a front runner is methanol. Methanol provides an important solution for the marine industry because it’s simple to create, and relatively easy to handle says Professor Martti Larmi, head of Energy Conversion research at Aalto University. It can be produced from biomass or renewable energy sources, but most importantly, it remains liquid at room temperature. However, despite impressive test bench results, the problem for pure HCCI is its narrow operating window, as it ditches both spark and fuel spray control of the ignition timing. There are a couple of challenges. “As the whole chamber ignites at once, it has a very rapid, dynamic combustion,” explains Pochet, creating a very large spike in cylinder pressure at higher loads. On the other hand, optimise combustion for medium to high loads “and at very low loads you end up with possible ignition failure”, says Pochet, adding that “cold start is also a problem”. As a result, other mechanisms are necessary to support or control ignition. Pochet explains that “you can play with boost pressure, fuel mixes, composition and intake air temperature through exhaust gas recirculation (EGR), which all affects the chemical kinetics of the combustion process”.
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Certainly, EGR is a useful control lever. At high load operation, the rate of reaction and heat release can be too much for the engine, explains BorgWarner’s Philip Keller. “The EGR brings largely inert gas, mostly nitrogen, but with water vapour and some CO2, back into the combustion chamber where it can slow everything down, lowering pressure and temperature peaks,” he says. It’s also useful at the other end of the range: “On light load operating points, where you need to reach a certain temperature to get the reaction, the exhaust gases can be a way to help initiate combustion.” Here, the turbocharger comes in useful: a variable geometry turbine can increase the exhaust backpressure, allowing greater retention of the hot residual gas. This helps mitigate another challenge: while HCCI engines’ lean combustion raises efficiency, they also have a lower maximum load level, necessitating a larger engine size for the same ‘oomph’. However, Keller also points out that “the efficient, lowtemperature combustion simply results in less energy to drive the air boosting process”. The problem is that further up the load range “homogeneous low-temperature combustion leaves you boxed into a corner”, explains Chris Kolodziej of the US’ Argonne National Laboratory (ANL). “More fuel drives higher reactivity, which necessitates higher EGR, but the greater the EGR portion, the more difficult it becomes to generate boost pressure from the exhaust-driven turbo”. It’s an uphill struggle; “and as EGR and boost pressure battle it out, you need a more capable air delivery system to meet the challenge”, says Kolodziej. Happily, there are now a number of ways to augment it, including applying an additional electrically driven compressor
8 HCCI/RCCI combustion gains a lift from technologies such as BorgWarner’s eTurbo which can augment or store exhaust energy
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