Green Engine Technology - Petrol Engines

Lean Burn Engine

Basically, engines which can operate in very lean air / fuel mixture are called "Lean Burn Engines". Japanese car makers, heading by Toyota, are the leaders in this technology.

Apparently, the leaner air / fuel mixture, the more frugal the engine is. But there are two reasons prevent conventional engines from operating in lean air / fuel mixture:

If the mixture is too lean, the engine will fail to combust.
Naturally, lower fuel concentration leads to less output.

Lean burn engines avoid these problems by adopting a highly efficient mixing process. They use special shape pistons, with intake manifolds located and angled matching the pistons, the intake air will generate swirl inside the combustion chamber. Swirl leads to more complete mixing of fuel and air, thus largely reduce the badly-mixed fuel particles, which will not be burnt in conventional engines. This enables more complete burning, not only reduces pollutant, but also allow the fuel / air ratio to be lowered from 1 : 14 to 1 : 25 without altering output.

Today, Lean Burn technology has evolved into Direct Injection, which is basically the former added with direct fuel injection. Toyota, Mitsubishi and Nissan all concentrate in DI engines development.

Direct Injection Petrol engine - Mitsubishi GDI

Mitsubishi is currently the leader of GDI (Gasoline Direct Injection) technology. It has already applied GDI in different engines, from 1.5-litre four to 4.5-litre V8. Now most of its production engines are GDI-equipped.

Mitsubishi claimed GDI consumes 20 to 35% less fuel, generates 20% less CO2 emission and 10% more power than conventional engines. How can it be so magical ? The following paragraphs will tell you its secret.

Theory of GDi

Gasoline direct injection technology is one of the branches of "Lean Burn Technology". What it differs with Lean Burn is the adoption of directly fuel injection system.

Direct fuel injection has been used in diesel engines for many years, but not in petrol engine until recently. Inherently, direct injection has two advantages :

Since the fuel is injected under high pressure directly into the combustion chamber, just before ignition by the spark plug, this allows the precise control of charge stratification vital to ignite ultra-lean air / fuel mixtures.
Direct injection also dispenses with the need for a throttle, so eliminating the pumping loss associated with drawing air around a conventional engine's butterfly valve.

In conventional engines, fuel injectors, even in MPi (multi-point injection) designs, the injected fuel pulverise in the intake port (near intake valves) before entering the combustion chambers. Why not directly inject into the cylinder ? because it is impossible to spread the fuel uniformally in everywhere. On the contrary, inject into the main entrance (intake port) assures all air mix with fuel in the same rate.

How can Mitsubishi applied direct injection without such problem? Let us look at the following diagrams:

Unlike conventional engines, GDI uses upright straight intake port, accompany with a concave-section piston surface, swirl air flow will be generated during compression stroke. When fuel directly injects into the combustion chamber, the swirl helps mixing air with fuel.

The fuel injector is another new feature. It pumps out the fuel at higher pressure, enables better pulverisation and more uniformal spread.

Fuel injection takes place in two phases. During intake stroke, some amount of fuel is "pre-injected" into the combustion chamber, cools the incoming air thus improve volumetric efficiency, and ensuring an even fuel / air mixture in everywhere.

Main injection takes place as the piston approaches top dead centre on the compression stroke, shortly before ignition. As seen in the above pictures, the concave-section piston concentrates more fuel around the spark plug, this allows successful ignition without misfire even when the air / fuel mixture is very lean. This explain why GDI can operate under fuel / air ratio of 1 : 40 under light load, which is even leaner than Lean Burn Engines. As a result, more complete burning is achieved.

More Power

Mitsubishi GDI engine has an extraordinarily high compression ratio of 12.5 : 1, this is perhaps the highest record for production petrol engine. The result is higher power output.

How can it prevent combustion knock under such pressure ? The secret is the pre-injection process. During compression, the heated air is cooled by the fuel spray, thus knocking becomes less easy to occur.

NOx emission

One of the few drawbacks of GDI engine is the higher NOx pollutant level. Luckily, a newly developed catalytic convertor deal comfortably with it. Nevertheless, USA and many developing countries cannot be benefited by it because their high-sulphur petrol will damage the catalyst.

Also see : The Problem of GDI in Europe

Direct Injection Petrol engine - Renault IDE

The Problem of GDI in Europe

As tested by a UK magazine, Mitsubishi Carisma GDI did not deliver higher fuel efficiency than competitors with conventional engines, very different to what the company claimed. This is simply not explainable until Renault launched its own direct injection petrol engine recently. In Renault’s press release material, there is implication that "a Japanese design" suffers from the relatively high Sulphur fuel in Europe, which is 150ppm compare with Japan’s 10-15ppm (although still a lot lower than that of the US). In Japan the GDI needs a special catalyst to clean the excessive NOx generating under ultra-lean combustion. However, the high Sulphur fuel could "pollute" the catalyst and makes it permanently ineffective.

Therefore the European Carisma GDI runs at much richer air fuel mixture than Japan’s sisters in order to reduce NOx, hence require only a normal Catalyst. While the Japanese GDI achieve a fuel / air ratio of 1 : 40 at light load, the European GDI can only reach 1 : 20 or so, compare to conventional engine’s 1 : 14. This greatly reduce fuel efficiency.

Another problem lies on different testing method between Japan and Europe. The test carried out by Transportation Department of Japan was done on a route and conditions consists of mostly light load operation, which suits GDI’s character (at light load GDI runs at 1 : 40 lean mode, otherwise at the 1 : 14.5 normal mode). European’s combined cycle test requires much more high load, high speed operation, thus resulting in mpg figures far worse than Japan’s claim.

Renault’s IDE (Injection Direct Essence)

Renault launched the first European direct injection petrol engine. It avoids the troubles encountered by Mitsubishi by implementing in a completely different way.

Instead of pursuing ultra-lean air / fuel mixture, they adopt ultra-high EGR (Exhaust Gas Recirculation). EGR, as mentioned here before, reduces fuel consumption by reducing pumping loss as well as by reducing the effective engine capacity during light or part load. At the lightest load, Renault’s IDE engine enables as much as 25% EGR compare with conventional car’s 10-15%.

How can IDE engine run at 25% EGR without failing to combust ? Thanks to the direct injection, which is at the center of the cylinder head in place of spark plug. The latter is relocated to the side nearby, very close to the injector outlet. The Siemens injector injects high pressure fuel (at 100 bar or 1450 psi) directly to the combustion chamber. As the inclined spark plug locates just at the path of the fuel spray, successful combustion is guaranteed even at 25% exhaust gas in the chamber.

Without the precise direct injection, conventional engines pulverize the fuel spray in the induction port thus enter the combustion chamber uniformally. As a result it is impossible to concentrate more fuel to the spark plug.

Depends on engine load, IDE runs at one of the 3 preset EGR ratios, among which the full load mode has no exhaust gas recirculation at all for the need of maximum power. Therefore, like GDI, running at full load saves no fuel. However, overall speaking Renault claims 16% reduction of fuel comsumption in real world, that is, according to the European test method. Well done.

Another to note is the enhance of performance. The 1998 c.c. engine output a solid 140 hp and a class-beating 148 lbft. As a comparison, the non-IDE but variable valve timing-equipped version output the same 140 hp but merely 139 lbft of torque. Not even the VVT matches the IDE.

Gain in performance is due to the increase of compression ratio to an unusually high 11.5 : 1 (GDI is even at 12.5 : 1). Like the Mitsubishi, a pre-injection in prior to the normal injection helps cooling the combustion chamber, thus raising knock resistance and enables a higher compression ratio.

Mercedes' 3-valve approach to cut cold start emission

Cold-start emission is the focus of attention in the latest engine designs. According to European newest regulation which will take effect in the year 2000, the emission during cold start period will be strictly controlled. In the past, catalytic converter used to provide satisfactory emission suppression after it has reached its operating temperature of around 300°C, but not during cold start.

To reduce the time taken to bring the catalyst to its operating temperature, apart from using close-coupled converter and pre-heated engine, Mercedes also tried to reduce the surface area of the exhaust port - by using a single exhaust valve in each cylinder rather than 2.

Mercedes 3 valves V6, one of the Ten Best Engines in AutoZine's engine award.

Many sees the transition from 4 valves to 3 valves as a reversal, but Mercedes claimed this is the only way for an engine with at least 6 cylinders to pass the Euro 2004 requirement (although I don't believe, neither do all other car makers). Reduced exhaust port surface area raises temperature for 70°C, vastly shortened the pre-heated period.

Of course, the drawback is some power loss. Therefore many other technology were employed to compensate - variable valve timing, variable intake manifold and twin-spark.