Lean-Burn Engine

A lean-burn engine refers to an internal combustion engine technology designed to burn petrol under conditions far leaner than the stoichiometric air-fuel ratio (an air-to-fuel mass ratio of 14.7:1). When the air-fuel ratio exceeds 18:1, it enters the lean-burn territory; in practical applications, this ratio can reach 25:1 or even exceed 40:1.

🎯 Why Pursue "Lean"?

Simply put, doing more with less fuel.

• Higher thermal efficiency: When a lean mixture burns, in-cylinder temperatures are lower and heat loss is reduced, enabling more chemical energy to be converted into mechanical work. Peak thermal efficiency in modern lean-burn engines can reach up to 45%, representing a significant improvement over conventional mainstream engines.

• Lower pumping losses: Under partial load, the throttle valve can be opened wider to reduce intake resistance, which inherently saves energy.

• Lower nitrogen oxide (NOx) emissions: Reduced combustion temperatures inherently suppress the formation of NOx from nitrogen and oxygen under high heat.

Overall, this technology delivers a 10% to 20% improvement in fuel economy, alongside enhanced power output.

⚙️ How to Achieve "Lean Burn"?

Ensuring a fuel-lean mixture combusts stably and efficiently is a tough engineering challenge. To overcome this, engineers have developed several key technologies:

1. Stratified charge combustion: Creating an "ignition-ready rich zone"
   This is the core strategy. Through specialised intake port designs or fuel injection strategies (such as Mitsubishi's GDI and Volkswagen's FSI), a relatively rich mixture (around 12:1 AFR) is formed around the spark plug for easy ignition, while the rest of the combustion chamber contains an ultra-lean mixture. Once the rich pocket is ignited, the flame propagates outward to the rest of the chamber.

2. High-energy ignition and strong tumble flow: Strengthening the flame

   • High-energy ignition system: Employing an ignition coil with energy exceeding 230mJ to produce a stronger spark, ensuring stable ignition even in ultra-lean mixtures.

   • Optimised intake and combustion chamber: Redesigned intake ports and piston crown profiles generate a strong in-cylinder "tumble flow" to accelerate flame propagation, pushing the stable combustion limit up to λ (excess air coefficient) > 2.0.

🚀 The "New Stage" for Lean Burn: Hybrids and Alternative Fuels

In the past, lean-burn technology struggled to gain widespread adoption in passenger cars due to emissions aftertreatment hurdles (particularly the difficulty of NOx reduction in lean environments). However, amid the current wave of hybridisation and alternative fuels, it has found fresh opportunities.

• Complementary synergy with hybrid systems: Dedicated hybrid engines operate within a narrower and more consistent operating band, bypassing the tricky control issues of lean burn during transient conditions. At the same time, the electric motor compensates for the lower low-end torque of lean-burn engines, allowing their high-efficiency benefits to be fully exploited.

• Empowering green fuels: Lean burn is an ideal approach for maximising the efficiency of alternative fuels such as methanol. For instance, in 2026, Horse Powertrain unveiled a lean-burn methanol range-extender system running on 100% pure methanol. Utilising a high-energy ignition system rated above 230mJ to achieve ultra-lean combustion, it delivers an impressive conversion efficiency of 2 kWh of electricity generated per 1 litre of methanol, while meeting future China 7 and Euro 7 emissions standards.