Mahle Powertrain, with partners Oak Ridge National Laboratories, Kohler Co., Louthan Engineering and Intellichoice Energy, has developed a prototype single-cylinder natural gas engine (above) that achieves 33% electrical conversion efficiency while powering a generator. The effort, under the aegis of the U.S. Dept. of Energy Advanced Research Projects Agency-Energy (ARPA-e), was launched in 2015. It realizes a 20% improvement over the most-efficient, currently-available such engines.

Key enabling technologies include Mahle’s Jet Ignition pre-chamber combustion system, along with aggressive friction reduction. The engine is intended for use as a compound heat and power (CHP) generator for residential use. Such distributed-energy systems can burn natural gas to produce electricity for a home while also using the waste heat for space and water heating. The potential energy efficiency for CHP systems is more than 80%, according to ARPA-e. Significant adoption of such systems have potential to enable dramatic reductions in primary energy use and CO₂ emissions.

“Electricity generated at the point of use is an effective way of combating the inefficiencies of centrally produced power,” says Mike Bunce, head of research for Mahle Powertrain LLC and principal investigator on the project. “CHP motors are a perfect solution for generating electricity and harnessing otherwise wasted energy for heating, for both primary power and heat generation or for use on an ad hoc basis during power outages.”

The obstacles to widespread adoption of personal generators by homeowners have been their high cost, their inefficiency and their reliability issues, according to Bunce, who co-authored a recent SAE Technical Paper detailing the engine’s development.

Mahle has already developed its MJI pre-chamber technology for automotive applications, but a generator has very different requirements that present unique challenges. While this single-cylinder application is a smaller displacement engine, the pre-chamber size and geometry do not shrink proportionally and finding the ideal configuration was a major challenge, Bunce said. Another effect of the pre-chamber is that fuel selection has a “disproportionally large effect on efficiency” compared to conventional engines, he added.

Natural gas is the preferred fuel for residential installations because the generator can tap the existing gas lines where available, for constant fuel supply. But natural gas mixes inside the engine differently than liquid gasoline, so Mahle had to refine its fuel injection for the pre-chamber. “Natural gas is more of a chaotic, less-predictable mixing event,” Bunce explained. “You need to aim the spray from the fuel injector a little differently.”

Mahle attacked friction on two fronts: by reducing the friction between the piston and the cylinder with the company’s EvoTech II low-friction, lightweight piston, and by reducing engine operating speed to just 900 rpm. Running at a constant, low speed reduces concerns about transient throttle response for example, but it creates its own challenges, according to Bunce. Running at such low speeds, variability of combustion cycles can be a problem, he said.

“One of the benefits that comes with MJI is that it produces good cycle-to-cycle combustion stability. MJI gives faster burning no matter what speed you’re operating. This is especially critical when using natural gas as fuel. “Natural gas is a tricky fuel,” Bunce said. “It lights off easily, but the combustion process is highly dependent on temperature.”  The natural gas combustion process can be so long that the flame cools and goes out during the combustion event, but the MJI pre-chamber helps prevent that.

Now, the partners have moved on to work on a larger-displacement version of the engine that would power bigger generators. To contain costs, Mahle and Kohler are pursuing ways to manufacture the cylinder head with the pre-chamber built into it rather than as a separate, bolt-on part. “We are getting closer to production feasibility,” Bunce said. The partners’ target is to bring a product to market within five years, he added.