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Reducing is Evolving: Downsized Engine Combines Low Fuel Consumption & Power

To upgrade an engine means increasing its size, right? No, bigger is not always better...

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2013/03/11

On Wrangel Island in the Russian Arctic Sea they once found a 2-ton woolly mammoth. Big enough but, considering that mammoths excavated in other areas have been on average roughly 6 tons, actually really small. What could have caused these "dwarf mammoths"? We can explain them using J. Bristol Foster's "island rule" theory from 1964, which says that large animals get smaller to preserve the species when they are living in an environment with limited resources.

The same thing is happening with cars. Generally we tend to think an engine is evolving if it's getting larger but actually there is another approach. We call this downsizing. It's when the engine displacement and number of cylinders are reduced. There are drivers, though, who like engines with lots of displacement and many cylinders, so they aren't disappearing for good. But make no mistake: Downsizing is now a major trend in the industry.

Not simply being small

In recent years carmakers have been adopting multiple strategies to make fuel consumption more efficient. And yet, in truth, just to make gas mileage better is a simple matter. All you have to do is cut the number of cylinders and the displacement in the engine. For example, you could change a four-cylinder engine to three cylinders, and, say, 1.5-liter displacement to 1.2-liters. If there are less cylinders and cylinder capacity, the engine becomes lighter, which directly leads to lower fuel consumption. Also, if we consider that an engine is a reciprocating machine, reducing the cylinders also then reduces friction. In other words, the movement of the engine becomes more efficient... and gas mileage is better.

There is a "but." If we just decrease the cylinders and displacement, the engine power also goes down and you have a vehicle whose engine must work harder. The way the car moves, the characteristic "feel" of the model, is changed; it becomes a different car entirely. What we want, then, is an engine that has both superb fuel mileage and is also fun to drive. Making this dream a reality is the concept behind the downsized engine.

The technology behind downsizing

How to make an engine that is smaller but without losing its driving performance? Use a supercharger. A supercharger is an air compressor that pumps pressurized air into an engine. When we think of turbochargers or superchargers, we might well have an image of technology that caters for guys who like fast cars. It's just for upping the power of a vehicle. It just guzzles gas purely in the name of speed. But actually the latest superchargers have enhanced turbine design and lowered drag, and are in fact a core piece of technology behind environmentally friendly and enjoyable driving. If we can utilize this kind of supercharger, it is possible to pump 1.5-liters of air into a 1.2-liter engine. In other words, superchargers in a small engine can give us the driving performance of a larger engine with more displacement.

Direct gasoline injection is also an important element of the downsized engine. In previous engines, fuel would be injected near to the cylinders and then be pumped in. On the other hand, with a direct injection gasoline (DIG) system, fuel is injected straight into the combustion chamber inside the cylinder. The merit of this is that then the gasoline vaporization cools the temperature inside the cylinder. In supercharger or turbocharger engines, the intake temperature is prone to heating up due to the air that is pumped into the cylinders at high pressure. It would conventionally then have to control the temperature by lowering the engine compression ratio.
Downsized Engine with Supercharger
downsizing_detail.jpg
In general, when engine displacement lowers, maximum power is reduced. This is due to a decrease in the volume of air being taken in and a reduction in the amount of fuel that can be burned at one time. The supercharger in Nissan's downsized engine pumps pressurized air into the engine (see arrows). The engine can compensate for the decreased volume of air taken in, enhancing the power of the engine and making driving a more comfortable experience.
Now let's look a little at this compression ratio. A gasoline engine pressurizes a vapor mixed with fuel and air, which it then ignites and burns off. Just like a water gun or airgun, stronger pressurization (raising the compression ratio) means combustion power increases. However, in a gasoline engine, more pressure leads to the combustion temperature becoming too high, giving you abnormal combustion and engine knock.

Gasoline direct injection, though, has a cooling effect, so the supercharger can pump in lots of air (i.e. air-fuel vapor) and burn off fuel while still keeping the temperature from going too high. This means we can increase the compression ratio without adverse effects. It was this deployment of a supercharger with a compatible direct injection mechanism that allowed Nissan to enhance both fuel mileage and the pleasure in the driving experience.

Engines evolve for high fuel economy while retaining driving pleasure

The "protagonists" of the eco car movement could be said to be hybrids and EVs. There can be no doubt about this in the long term, but the truth is that hybrid cars have complex mechanisms that make them expensive, while electric vehicles require specialist social infrastructures to be put in place before they become truly widespread. In addition, it is in the newly emerging nations that automobiles will be increasing in the coming years. If we consider the actual situations in each country or region, surely it is realistic to say that for now, gasoline engine vehicles will be mainly on the rise.

Our environmental concerns like global warming cannot only be tackled by the spread of hybrid vehicles and EVs. It is also essential that we enhance gasoline engines in order to improve fuel efficiency. Nissan's downsized engine combining both low fuel mileage and driving comfort is just one example of the innovative concepts surely soon to appear in the near future.