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”.

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, low-temperature 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 such as BorgWarner’s eBooster, or their eTurbo design where a motor-generator is sandwiched between the turbine and compressor on the turbocharger. Interestingly “when there isn’t enough energy in the exhaust, it can pull energy from the electric system… and it can be used in the other direction so that excess exhaust energy can be fed back and stored.”

So while Kolodziej points out “the energy still has to come from somewhere”, there is some potential recuperation from these devices.

Could it fly (or swim) into the marine industry? Well, it seems that given all this potential for high efficiency and low emissions, some may be seeing a future for HCCI combustion. Certainly, Pochet concludes that it may find a home in steady-output shipboard applications.

RCCI

There are other approaches for those that want a broader operating window and higher power density.

Reactivity Controlled Combustion Ignition (RCCI) accomplishes this by introducing another fuel with different autoignition properties at a later point in the cycle. Instead of a completely homogenous mix, this results in a ‘stratified’ charge in the combustion chamber: extending or separating ignition into two – or more – events effectively draws out and dampens down the pressure and temperature spikes.

As it “allows for better control of the ignition process and combustion duration… you can add more fuel, increasing power density”, explains Pochet.

But, isn’t all this quite difficult to achieve? Not necessarily, says Larmi, explaining that RCCI tends to use relatively low injection pressures for the low reactivity main fuel: “There are two principal ways of injecting methanol in a four stroke engine, one is to modify the cylinder head, but the other is simpler, just put it into the upstream port fuel injection channel, that’s the easiest and cheapest method.”

Moreover, he explains “as methanol is quite easy to handle, if you’re going to modify an engine for RCCI methanol combustion there is typically room for integrating the methanol injector in the intake channel”. Further, Larmi believes that it could even be picked up by the manufacturers to create a retrofittable installation kit.

However, for higher loads, even methanol’s relatively low reactivity can generate aggressive combustion rates under homogeneous conditions, prompting a look at other fuels including natural gas and diesel combination. It’s also beneficial to have two fuels of very different reactivity, adds Kolodziej: “The further apart their ignition chemistry, the wider the spectrum for control.”

As a result, the nature of this second fuel may require some thought: “For the high reactivity fuel there are a lot of options, including biodiesel and synthetic fuels, but the more paraffinic types might be better as they generally have a higher cetane number”, says Larmi. “It’s also possible to use DME – although that will need more advanced, possibly more costly injection equipment for this low viscosity fuel.”

There are still challenges. Although this particular RCCI approach is excellent for low NOx and soot, fuel residing near the cooler combustion chamber walls can leave you with hydrocarbon and CO emissions, though again, this is treatable with catalytic converters.

But most importantly, while mid-loads will favour more of the lower reactivity fuel, at the bottom of the load spectrum the balance doesn’t lend itself to stable, robust combustion. Moreover, at high loads, this well-mixed fraction can too quickly auto-ignite. Therefore, an increasing load forces a reduction in the amount of port injected low-reactivity fuel, with combustion potentially shifting over to as much as 90% of its high-reactivity counterpart. As Kolodziej explains, “it means going from a kinetically driven RCCI process to using a more responsive direct injection of high-reactivity fuel to initiate ignition”. Unfortunately, this “looks more and more like diesel combustion”, he adds, with a consequent return to the old emission problems.

Therefore, there has been a look into extending kinetically-driven RCCI to higher loads by bringing in Variable Valve Timing (VVT), which, amongst other things, can reduce the effective compression ratio. This allows retaining high fractions of the low-reactivity fuel without aggressive combustion, extending the reach up the load range.

Further, by maximising the portion of the well-mixed low reactivity fuel at high loads, it displaces the quantity of fuel participating in the later direct-injected high reactivity fuel combustion, reducing particulate and NOx output.

POTENTIAL

It appears there is, at the moment, a groundswell of interest in the potential for HCCI and RCCI engines.

But, pushing open the operating window has given rise to a very diverse set of part-load approaches which tailor combustion in different ways. This includes everything from two-stage ignition and one fuel, to a single, more extended combustion event from two different fuels or variations with multiple inputs. Pochet adds: “There are also more exotic techniques, for example, temperature stratification through direct injection of water.”

A single-fuel strategy could be extended at low load by transitioning from one mode to another. For example, HCCI could get over some of its cold-start issues by beginning operation in spark or advanced CI mode.

Further, ANL (among others) is also researching Gasoline Compression Ignition (GCI) which overcomes load limitations by well-timed, direct injection of low reactivity fuels to stratify the charge.

But other technologies, currently developing in parallel, may help make this a reality. For example, a hybrid configuration could allow HCCI or RCCI combustion to stay within a restricted operating envelope, letting an energy storage component pick up the other, more problematic loads.

Moreover, the diesel-like RCCI approach can be developed further into direct injection of both high and low-reactivity components: this would be very timely considering the marine industry’s current involvement in dual-fuel strategies.

Taking this further, “direct-injection of both fuels gives you the potential for altering your fuel blend and in-cylinder mixture stratification on the fly”, says Kolodziej. That promises to be very interesting…

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