‘Lean Burn’ Petrol Engines

It’s been over ten years since the first ‘stratified combustion’, directly injected, or ‘lean burn’ petrol engines started to materialise. The latest lean-burn engines’ emissions could not be measured for MOT by the then current 4-gas analysers. In this edition GEA’s Dave Garratt explains how lean-burn engines work, and in May 2012’s edition he will explain why our emissions equipment had to be modified.

History:

The first car to use such technology was the Mitsubishi Charisma, which they called ‘Gasoline Direct Injection’ or GDI, which sold more than 37,000 vehicles in the first six months… by 2001 they had sold 1,000,000 worldwide!

This was considered to be the most significant engine technology innovation for over 20 years, and it didn’t take long for other manufacturers to follow and by the year 2000 Volkswagen launched their first such engine, called Fuel Stratified Injection or FSI. Both manufacturers used direct injection and stratified combustion with these engines. Renault launched their system known as IDE Injection Direct Essence in 2001 and Mazda, Nissan, Peugeot and BMW have all followed suit.

Whilst direct injection technology alone increases torque and power making them as much as 15 percent more economical, its greatest advantage is that it introduces the possibility of running the engine using ‘stratified combustion’ .

Stratified Combustion

Stratified combustion is traditionally used on diesel engines where the fuel is injected into a combustion chamber already partially charged with swirling air. As diesel fuel is injected, it immediately ignites causing it to instantly mix and burn, but only in the centre of the cylinder.

The areas within the cylinder outside the ‘slug’ of burning fuel remain charged with pure air, acting as thermal insulation between the burning fuel and the cylinder walls, allowing more heat conversion into power, reducing waste heat which needs to be cooled. In addition, the overall air to fuel ratio is much higher; giving the stratified method clear advantages in both economy and emissions.

Petrol engine restriction

Using the stratified method alone on petrol engines is not a viable option. This is mainly because petrol has different properties to diesel, resulting in power and torque loss at the weak mixtures resulting from stratified combustion, particularly with the engine under load. However, the emission and economy advantages remain. The ‘trick’, is to gain the benefits of stratified combustion in petrol engines by arranging for it to only ‘kick-in’ when the engine is ‘off load’, and revert to normal petrol/ignition combustion when the engine is loaded – something made possible with the advent of computerised Electronic Control Units (ECU), which can regulate the fuel injectors and air flow differently under differing operating conditions. This improves both emissions and economy whilst maintaining good performance when it’s needed.

Combustion – conventional and ‘lean burn’

To achieve both types of combustion, the design of the engine’s combustion chamber is unique. At first glance, it’s like a diesel engine. The injector is positioned just under the inlet valve and the piston contains a large recess on the intake side, which swirls the fuel. On the outlet (exhaust) side of the piston is a smaller recess, swirling the induced air. The inlet manifold has one throttle valve and one tumble flap per cylinder. The manifold is divided into an upper and a lower channel and when the flaps are actuated, the lower channel is closed.

Stratified ‘lean burn’ mode…

When operating in stratified mode the electronic throttle valve is held continuously open by the ECU, with the tumble flaps positioned to block off the lower channel. This causes the incoming air to flow mainly over the upper edge of the inlet valve, which swirls the induced air so the air is rotating.

As the piston starts to move upward on its compression stroke, the rotating air movement is intensified by the ‘swirl recess’ in the top of the piston. As the piston approaches top dead centre the air is continuously swirling similar to a whirlwind. It’s at this moment that the fuel (much less than for a normal petrol engine ignition cycle) is injected directly into the piston’s fuel recess, causing the small slug of fuel to swirl in the opposite direction to the swirling air, fully mixing the fuel and air.

As the piston nears the last quarter of its stroke, the swirling air, together with the swirling fuel, causes a small ball of ignitable charge to be formed around the spark plug. The ball is known as the ‘stratified charge’, which is then ignited creating a small fireball directly beneath the spark plug. This then heats and acts upon the air in the rest of the cylinder which drives the piston down.

Note that when running in stratified charge mode, the electronic throttle valve is almost fully open. The engine’s power is controlled by regulating the amount of fuel injected – just like a diesel engine.

Normal combustion (homogenous) mode

So how does the engine switch between combustion modes? The first action to take when changing to the normal mode is to reposition the throttle valve to match the position of the accelerator pedal. At the same time the tumble flaps are deactivated to direct the induced air through both of the inlet channels.

As the air is drawn into the cylinder (during the induction stroke), the fuel is injected directly into it. Injecting at this earlier part of the engine cycle allows the fuel time to fully mix with the air, which results in the pre-mixing of the whole charge of fuel/air within the cylinder.

The piston then returns on its compression stroke and just before the piston is at top-dead-centre the charge is ignited.

It’s the engine management’s job to decide which mode is best suited to the driving conditions, but the rules are straightforward. In cruise, and at light throttle, the engine runs mainly in stratified mode, and under load or acceleration, it’s switched to normal operation. The switch-over occurs so smoothly that it’s undetectable to the driver.

When operating with stratified combustion the Lambda value of the engine is 1.6 – 4.0, which is so weak that the charge would fail to ignite if it was mixed homogenously. Therefore, at part throttle, there’s a potential fuel saving of up to 15 percent and yet by automatically switching to homogenous when needed, the engine maintains an impressive power output.

However this does have an effect on the emissions due to the weak mixture during when operating in the stratified combustion burn mode.

In the next issue of MOT Testing Dave Garratt will explain how this affects the engine emissions, and in turn how it is measured for the MOT.

MOT Workshop Magazine