The less the better

In this post I will share with the readers the recent technological advances aimed at making the internal combustion engine more fuel efficient.

Three years ago I bought a brand new Suzuki Swift with a 1372cc normally aspirated petrol engine. It is such a fantastic car that I was surprised to learn Suzuki are about to improve it by putting in a new “DualJet” 1242cc engine which will use much less fuel. Before we get into how they managed to achieve it let us look at what physical and engineering realities impact on the fuel economy.

In essence internal combustion automotive engines convert the chemical energy in carbohydrates into mechanical energy required to move the vehicle. In any such process some energy will be wasted – mainly as heat which is dispersed into the atmosphere. There are certain thermodynamic rules which govern the overall efficiency of an engine which, at the practical level, mean that for example:

  • Higher compression of fuel in cylinders leads to better thermal efficiency
  • Smaller engines have lower friction and inertial losses which minimises waste
  • Turbo-charged engines recover some of the wasted energy but cost more to build and service

I will now look at how the recent engine designs by various car makers take advantage of one or more of the above principles.

The EcoBoost family of Ford engines are small displacement turbo-charged petrol units. Since their turbo only activates when the car is pushed hard most of the time they operate as economical small engines with low friction and inertial losses. The additional savings come from the fact that the EcoBoost engines are lightweight, reducing the kerb weight of the car. What a brilliant idea – a well engineered compact and lightweight engine with more power available through turbo-charger which only kicks in when needed. Here is the cut-out of an EcoBoost engine, with the turbo on the right.

2011-ford-mondeo-ecoboost-engine

General Motors chose a different way of making their engines more economical, known as variable displacement. Would it not be great to have a small engine for cruising at modest speeds and a larger one for uphill stretches and acceleration off the mark? Well, this is what Active Fuel Management effectively provides. The 5.3L engine in Chevrolet Silverado switches off four of its eight cylinders when the power demand is low. If the pedal is pushed hard all cylinders seamlessly activate and full power is available. The fuel saving is less impressive than in the EcoBoost engines but there are no extra costs associated with the turbo. Great.

Hybrid petrol/electric engines utilise a somewhat similar concept in that a small displacement internal combustion engine is supplemented by a battery powered electric power train. When driven at normal speeds Toyota Prius is powered by a small, economical petrol engine. During uphill acceleration the electric motor activates giving the extra push when required. The extra fuel saving is provided by regenerative breaking. This is when the kinetic energy of the decelerating car is converted into electric power and stored in the batteries to be used during re-start. Well done, Toyota.

ToyotaHybrid2

But what have Suzuki done to make the engine in my Swift obsolete? They went for a higher compression ratio. Instead of the usual 10.5 their new “DualJet” unit compresses the fuel/air mix in the cylinders by a factor of 12.0, which gives higher thermal efficiency. This was not easy to achieve as petrol vapours tend to self-ignite when compressed – the phenomenon known as “pinging”. But Suzuki engineers found out that by improving the engine cooling of the engine block, re-shaping the combustion chamber and using two injectors per cylinder they can suppress pinging. This resulted in a small and economical engine which, according to the official figures, reduced the fuel consumption of the new Swift to 4.3L/100km. Click on the photo below to watch a short movie explaining the DualJet concept in detail.

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There seems to be no end to progress in the world of technology.

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