ClearFlame, whose technology allows diesel engines to run on more climate-friendly fuels, said the Series B round was also backed by WIND Ventures, the venture arm of Chile's COPEC, and existing investor Breakthrough Energy Ventures.

As the world's biggest automakers race to electrify passenger vehicles, the engines most commonly used by trucking companies, trains, farmers and utilities remain largely powered by diesel, a fossil fuel that also causes heavy air pollution.

To help accelerate the shift to a low-carbon economy, ClearFlame modifies the engine design by switching out 10-15% of the engine parts to allow it to run on a variety of fuels, including ethanol and ammonia.

"Effectively...it's a higher-temperature combustion process It's very easy to integrate into the manufacturing and the maintenance process," which helps keep costs down, said ClearFlame CEO and co-founder BJ Johnson.

The $50,000-$60,000 cost of retrofitting a Class-8 truck, for example, could be recovered in 12-15 months, he said, which would likely prove attractive to the many companies which have pledged to cut fleet emissions.

"Our carbon impact per mile is actually lower than that of driving an electric vehicle; the fuels we use are cleaner than the electrical grid," Johnson said, making the switch more cost-effective.

Diesel accounts for around 26% of carbon emissions from the transport sector, ClearFlame said, and is also responsible for particulates and black soot. A fuel like ethanol can cut emissions by 45-50% and soot by more than 90%, Johnson said.

The company, which already counts U.S. farm equipment maker Deere & Co as an investor, is working with five of the biggest U.S. truck fleets to pilot the technology, and looking at branching out into the mining, agriculture, and power generation sectors, Johnson said.

Mercuria's Boris Bystrov said in a statement that its investment reflected a belief that ClearFlame's technology can "economically decarbonise the heavy-duty industry".

(Reporting by Simon Jessop; Editing by Kirsten Donovan)

By Simon Jessop