Will we see a return to petrol engines?

It’s accepted wisdom that there’s no alternative to the heavy truck diesel for linehaul tasks. But two highly-qualified US researchers disagree and have come up with a petrol-fuelled alternative.

As diesel engine makers struggle to meet increasingly demanding emissions laws, vehicle buyers are looking for lower-cost, less maintenance-intensive alternatives. Could the petrol engine make a return to heavy-duty trucking?

Several years ago, MIT’s Daniel Cohn and Leslie Bromberg, principal research engineer at the Plasma Science and Fusion Center and the Sloan Automotive Laboratory, took on the challenge of designing a low-emission, fuel-efficient replacement for the diesel engines traditionally viewed as the only viable powerplants for heavy-duty trucks.

This research was supported by the Arthur Samberg Energy Innovation Fund of the MIT Energy Initiative and the researchers were photographed by Stuart Darsch, of Boston.

Diesels require increasingly more complex emission-control systems that are now second only to tyre maintenance in US fleet operating costs.

Powered largely by diesel engines, trucks are now the largest producer of nitrogen oxide (NOx) emissions in the transportation sector, contributing to ground-level ozone, respiratory problems and premature deaths in urban areas.

Today’s heavy-duty diesel engines provide fuel efficiency and high power and torque, making them ideal for long-haul, high-mileage commercial vehicles, but finding another option is critical, said Daniel Cohn.

“We need to replace diesel engines with other internal combustion engines that are much cleaner and produce less greenhouse gas.”

Using computer simulation analysis, Cohn and Bromberg have designed a replacement, half-sized gasoline-alcohol engine that should be not only cleaner, but also much lighter, lower-cost and higher-performing.

During normal gasoline spark ignition (SI) engine operation, the process of translating the combustion of gases into torque at the wheels progresses smoothly, until there’s a need for high-torque operation, for example, to pull a heavy load at high speed or up a hill.

Under this increased load, pressures and temperatures inside the cylinders can rise so much that the unburned combustion gases spontaneously ignite. The result is pre-ignition or ‘knock’ that’s characterised by a loud, metallic clanging noise and can destroy the engine.

For a heavy-truck petrol engine the task is more difficult, but Cohn and Bromberg dealt with that problem using alcohol. When the spark ignition engine is working hard and knock is imminent, a small amount of ethanol or methanol is injected into the hot combustion chamber, where it quickly vaporizes, cooling the fuel and air and making spontaneous combustion much less likely.

With concern about knock removed, the researchers took full advantage of two techniques used in today’s passenger cars: firstly, they used turbocharging, but at higher levels and, secondly, they used a high compression ratio.

Also, using an ideal air-to-fuel (stoichiometric) ratio, a three-way catalyst was sufficient to clean up the engine exhaust. In addition, because the spark ignition engine can’t use the excess air that’s in a diesel, the total volume of its cylinders could be smaller.

“Because of that difference, you can replace a diesel engine with an SI engine about half as big,” said Leslie Bromberg.

With that reduction in size comes an increase in fuel efficiency and cost. The cost of the gasoline-alcohol engine, including its exhaust-treatment system, should be roughly half that of the cleanest diesel engine.

But how does the proposed half-sized petrol-alcohol SI engine compare with today’s cleanest full-sized diesel on efficiency and power?

The MIT analysis assumed that a Cummins B6.7-litre petrol-alcohol-converted engine’s compression ratio was about the same as in the company’s 12-litre diesel engine.

Because of its faster operation, the small engine can produce almost 50 per cent more power than the diesel can.

While the gasoline-alcohol engine is somewhat more efficient than the diesel at high torque and less efficient at low torque, in general the small petrol engine is about as efficient as the diesel.

The accompanying graph shows efficiency, ethanol proportion and peak torque – 2000Nm isn’t bad from a 6.7-litre SI engine is it?

However, as more torque is required, knock becomes more likely, so more ethanol is needed.

As an example, the researchers considered a trip taken by a long-haul, heavy-duty vehicle that requires high torque most of the time. Depending on the compression ratio, ethanol could make up 20 to 40 per cent of its total fuel consumption. In contrast, a delivery truck might operate at low torque most of the time and do just fine with ethanol as 10 per cent of its total fuel over a driving period.

One way to reduce ethanol use would be to dilute the ethanol with water. Using the knock model, Cohn and Bromberg determined that knock resistance is actually higher when water makes up as much as a third of the secondary fuel.

“And in some cases where you don’t need any ethanol for antifreeze, you might be able to run with water alone as the secondary fluid,” said Cohn.

The MIT researchers are no pie-in-the-sky academics. Their SI engine R&D is mirrored by Cummins’ own efforts.

I asked Cummins if that company was involved with the MIT research project, because it’s well known that Cummins, through its Westport subsidiary, has been producing SI engines, mainly fuelled by natural gas, for years. Cummins declined to comment on the MIT initiative, other than to say that it continues to investigate advanced petrol-engine technology.

In late 2017, Brett Merritt, executive director for on-highway business at Cummins, stressed the company's technical expertise and highlighted its wide product portfolio, noting the company is investing $700 million annually around the world in research and development.

Although Cummins believes diesel will remain the power source of choice for North American trucking for decades to come, the company is continuing to develop and invest in alternative fuel engines and technologies.

Cummins has been working on high-efficiency spark-ignited technology "that can deliver diesel-like performance and durability" across a range of liquid fuels from ethanol to methanol and petrol and can meet "the most stringent emissions requirements".

At its Columbus, Indiana, technical facility, Cummins operates 88 test cells running diesel, natural gas, petrol, ethanol, hydrogen, propane and biodiesel engines and has also invested in and continues to explore fuel-cell power.

It’s an exciting time we live in, and we’ll keep you posted on further developments.