BYD has launched its DM-i hybrid technology for plug-in hybrid vehicles (PHEVs) with the 1.5-litre Xiaoyun engine.

The new, highly efficient PHEV engine was built specifically for DM-i hybrid technology. With brake thermal efficiency (BTE) of 43%, it is the world's highest thermally efficient production petrol engine, BYD claimed. The engine easily meets China's newest national emissions standards.

In the Chinese government's recent New Energy Vehicle Industry Development Plan (2021-2035), NEVs are expected to account for 20% of the total sales of new vehicles in China by 2025. Among these, the path for PHEVs shows immense potential, BYD said. BYD's dual-mode (DM) hybrid technologies – DM-p and DM-i – further enhances the competitiveness of PHEVs against traditional fuel vehicles.

The DM-p platform focuses on performance while DM-i hybrid technology brings further reductions in fuel consumption, faster acceleration, smoother and quieter ride and more environmentally-friendly electric power.

Eric Li, deputy general manager of BYD Auto Sales, said, "BYD has always insisted on self-reliance to build its core technology. With our advantages in new energy technologies, we have achieved major breakthroughs in several key areas. The DM-i hybrid will rigorously accelerate the replacement of traditional fuel cars with new energy vehicles."

The Xiaoyun engine has an ultra-high compression ratio (CR) of 15.5, an increased B/S ratio, an Atkinson cycle for improved combustion efficiency, an exhaust gas recirculation (EGR) system, a series of friction-reducing measures and an engine control system that is optimised for high thermal efficiency targets.

It takes full advantage of the electrification of plug in hybrid models, electrifying accessories and removing the traditional front engine accessory drive system, further reducing wear and tear and improving efficiency.

BYD has also implemented split cooling technology. Through on-demand cylinder head and cylinder block temperature measurements, this enables precise and accurate cooling to reach optimal running temperatures. By mitigating heating losses, this shortens the length of warming up the engine after a cold start by 15-20%, reducing fuel consumption and carbon emissions.

To improve noise, vibration, and harshness (NVH) performance, the engine has optimised designs for the crankshaft, bearings, cylinder block, intake manifold, oil pan, timing cover, cylinder head cover and other components of the plug-in hybrid system.