Diesel breaks 50% efficiency barrier

By Categories: NewsPublished On: Saturday 31 October 2020

Chinese engine manufacturer Weichai Power claims to have broken a significant barrier in truck engine design by producing a 13-litre unit with a thermal efficiency exceeding 50 per cent.

Thermal efficiency is the measure of work done by the heat generated. In automotive engine terms this is the heat generated by combustion, measured against the power delivered by the engine’s crankshaft. Air-pumping losses, internal friction, parasitic drag from accessory systems, sub-optimum oxygen/fuel ratios, and rejected heat (lost to the cooling system and exhaust) all reduce thermal efficiency.

An additional factor is NOx: engines running at peak thermal efficiency and optimum combustion ratios tend to produce high NOx outputs because the peak temperatures reached in the combustion chamber burn atmospheric nitrogen, producing oxides of nitrogen. Exhaust gas recirculation reduces NOx by obstructing combustion with additional CO2. NOx exhaust emission limits have tended to constrain recent developments in engine efficiency.

Automotive petrol engines have thermal efficiencies typically as low as 25 to 30 per cent, rising to 35 per cent for those employing direct injection. Diesel engines record greater efficiencies thanks to their higher compression ratios. The greater the efficiency, the more work the engine can do on a given quantity of fuel.

Modern truck engines in the 13-litre class tend to have peak thermal efficiencies of about 46 per cent, although very large low-speed two-stroke diesel engines used in marine applications, such as the MAN S80ME-C7, have thermal efficiencies exceeding 54 per cent.

Weichai Power, China’s largest engine manufacturer for the commercial vehicle segment with an output exceeding a million engines a year, said its development of the 12.9 straight-six began in September 2018, and is part of a co-operation with German technology giant Bosch dating back to 2003.

Tan Xuguang, chairman of the Weichai Group said: “Bosch offered great support on the breakthrough and unprecedented achievement of 50 per cent efficiency in Weichai commercial-vehicle diesel engine. We’re very pleased to be continuing our highly successful partnership with Bosch.”

Bosch supplies its modular common-rail diesel-injection system for the engine. It can be used for pressure levels from 1,800 to 2,500 bar. High injector flow rates make it possible to optimise the combustion strategy and achieve high engine performance. Depending on the demands it is subject to, the system has a life of up to 1.6 million kilometres.

For its part, Weichai created five proprietary technologies: advanced field synergy combustion technology, harmonious design technology, exhaust energy distribution technology, subzone lubrication technology and WISE control technology,

Carefully matching air flows, fuel injection, and combustion-chamber shape improved the engine’s combustion speed by 30 per cent.

Bosch chairman Dr Volkmar Denner said: “In increasing the efficiency by four percentage points, we have jointly reached a new milestone. Even though the diesel engine is nearly 130 years old, its development continues.”

He added that, although Bosch is heavily involved in alternative driveline technologies, when it comes to commercial vehicles, and above all when heavy loads have to be transported over long distances, the diesel engine will remain the preferred choice for the foreseeable future.

“For this reason, Bosch and Weichai aim to steadily refine this technology with an eye to protecting the climate and environment,” he said.

Certificates confirming thermal efficiency of 50.26 per cent for the new engine were awarded by Germany’s TÜV SÜD, and China’s national internal combustion engine testing organisation, China Automotive Technology and Research Center Co. Ltd.