The first 200 seconds count when starting an engine. That’s because for many applications, more than 50% of the tailpipe emissions in an FTP-75 are produced in the first 200 seconds of operation after a cold start.
To help meet global regulatory standards and reduce cold-start emissions, Achates Power has developed a patent-pending temperature control strategy for achieving higher exhaust temperatures during the catalyst light-off phase than are possible with conventional, four-stroke diesel engines.
This strategy, which will be introduced at the CRC 22nd Real World Emissions Workshop, takes advantage of several unique characteristics of the opposed-piston, two-stroke engine (OP2S), creating increased exhaust temperatures that:
- Yield shorter catalyst light-off times
- Reduce cold-start and cycle CO2 emissions
- Improve overall fuel economy
With new, and stricter, worldwide emissions and fuel efficiency regulations (such as LEV III and CAFE 2025), engine developers are forced to compromise overall efficiency in order to meet required tailpipe targets. One way to do this is by altering engine combustion to manage engine-out emissions. Another way is by providing exhaust temperatures that enable the catalysts to perform the final emissions conversion to meet the tailpipe target. But, the least detrimental strategy to fuel efficiency is to use different engine settings after a cold start to quickly increase exhaust temperatures and achieve the fastest catalyst light-off time. Once the catalyst is lit, the engine-out emissions level can be relaxed and the settings adjusted to optimize for fuel consumption.
While gasoline engines have typically relied on cold-start strategies to quickly generate significant heat, conventional diesels haven’t been able to achieve high exhaust temperatures because of the trapped conditions required for stable diesel combustion. OP2S engines, on the other hand, can generate trapped conditions to achieve stable combustion and high exhaust temperatures simultaneously without specific additional hardware. This makes it possible to tailor the combustion and exhaust gas temperatures to satisfy the requirements of the aftertreatment system for faster light off. It also allows the Achates Power OP2S to overcome the traditional disadvantage of diesel versus gasoline engines with respect to catalyst light-off times as well as produce dramatically superior fuel efficiency over four-stroke diesel engines.
Although this strategy is specific to catalyst light off, it can also provide benefits during more extreme cold starts (ambient temperatures below 0°C), light-load operation and under all speed and load conditions for active Diesel Particulate Filter (DPF) re-generation.
To quantify the effects of this strategy, tests were run on a single-cylinder engine and the results extrapolated to post-turbine conditions using a multi-cylinder model. We found that running the engine with low modified delivery ratios in combination with adequate intake manifold boost pressures and a late injection technique produced a turbine outlet temperature of 367°C (specific exhaust enthalpy of 378 KJ/kg) at 1000 rpm and only 0.53 bar BMEP. This was achieved while only producing 27 ppm of NOx, which corresponds to a humidity-corrected NOx flow rate of 0.9 mg/s.
The flexibility offered by the OP2S engine’s large range of trapped conditions is a key enabler to control emissions, particularly during cold-start conditions. To learn more about this temperature control strategy, attend the Achates Power presentation at the CRC 22nd Real World Emissions Workshop on March 28 at 12 p.m.
It would be nice to clarify the term “trapped conditions of diesel engines” a bit for the layman, and why those conditions prevent conventional diesels from achieving high exhaust temperatures.
“Trapped conditions” refers to the properties of the charge that are in the cylinder at the beginning of the closed portion of the cycle, or when both the intake and exhaust ports are first “closed off” and no charge enters or exits the cylinder.
The opposed-piston, two-stroke (OP2S) and its air system configuration has the capability to provide a wider range of trapped conditions because the pumping is controlled independently of the crankshaft speed. The base air system layout for the OP2S engine includes an electronically adjustable supercharger.
Unlike diesel combustion, there are several characteristics of gasoline combustion that enable it to more quickly reach the high exhaust gas temperatures required for catalyst light-off. In a gasoline engine, a certain balance of air and fuel (called stoichiometric) is needed, while diesel engines tend to run leaner than stoichiometric (more air than fuel). Diesel lean combustion is more efficient, due to properties of combustion chemistry, but exhaust gas temperatures will be lower, as the extra air not involved in combustion chemistry serves to dilute, or cool, the exhaust gas temperature. Before aftertreatment was required, the colder exhaust was not an issue. But with aftertreatment systems, a certain temperature range is necessary to be efficient. The problem with just trying to operate richer in a diesel engine is the risk of combustion instability and high smoke. However, with the OP2S engine, the trapped conditions can be adjusted to achieve lean conditions for good combustion and low flow rates for high exhaust temperatures at the same time.
I have just forwarded an email and missed your article, the way to resolve the problem is to make the engine dual fuel and to take advantage of an increase in exhaust temperature by using a pre combustion or afterburner using a very small amount of petrol , alcohol or other volatile hydrocarbon and feeding the now increased temperature exhaust to a turbocharger inter cooler, As Albert said, just make it complicated enough to function at optimum efficiency!