Gone are the days when a turbocharger or a supercharger was only used in cars that were purposely built for racing or for higher torque and horsepower applications. Today, the turbocharger is widely used all around the world, amidst the aim of reducing emissions. By using a turbocharged engine car manufacturers are able to make the engine capacity or size smaller while still maintaining those high-power figures. Back in those days’ turbochargers were only limited to bigger and more expensive cars since the cost of making turbochargers and its accessories was high and there were minute problems with turbochargers like turbo lag. Now the industry has flourished a lot and turbochargers are becoming more common. Car manufacturers have also started putting turbochargers on small cars, as small as 660cc ones. While there are multiple benefits of using a turbocharger, there are downsides too on smaller engines, mainly fuel economy, which is the primary concern here in Pakistan.
There are tons of 660cc cars being imported to Pakistan, which are turbocharged that include Daihatsu Move, Suzuki Wagon R Stingray, Suzuki Alto Turbo RS, Daihatsu Copen, Mitsubishi Ek Wagon, Suzuki Every, and some others. So, before we start discussing why smaller turbocharged engines do not get good fuel economy as mentioned by the manufacturer, let’s take a look at how a turbocharged engine works.
So, the basic idea behind a turbocharger is that it has two parts. One part is connected to the exhaust and the other part is the turbine, which sucks in more air. So, when the car is running the exhaust gasses spool up the turbo which in turn draws more air in. For the sake of argument, let’s say that the atmospheric pressure inside the engine is 15psi and using this turbocharger we are able to add in 7.5psi of additional boost, which means we get 50% additional oxygen inside the engine and as a result, we will inject more fuel, with that more air and more fuel burning we get somewhat around 50% more power. So, the idea behind the turbocharger is very clever. You are able to get small displacement engines but still get more power out of them.
So, the thing with turbochargers is that you can’t have both; either you will get better fuel economy or power. There are ways to get both at the same time but those are highly unlikely cases. The main problem with turbocharged engines is knock, so manufacturers have to come up with a way on how to eliminate that knock. So, what is the knock? As that piston is compressing air and fuel mixture on its way up to the top of the cylinder, your spark plug fires and ignites that air-fuel mixture but the temperature and the pressure is very high since the engine is turbocharged, so you may have pockets where that air-fuel mixture is already hot enough that it ignites on its own which is also known as premature combustion. So as this happens you have two ignition sources and these interfere with each other, worst case scenario is that you may even damage your engine if there is a significant engine knock. So, a turbocharged engine is adding in more air, adding in more fuel, resulting in more pressure in each of the individual cylinders of the engine, meaning more temperature as well and all of this combined gives you a higher probability of knock.
So, what do manufacturers do to eliminate knock, so you can still have high boost levels and high power without the knock? You can hinder the ignition timing but that means that the engine will not be as efficient nor will you have higher power so you can’t do that. You can lower the compression ratio but that also means less power and overall lower efficiency. Generally, car manufacturers use lower compression to eliminate the possibility of knock. Now the main reason that you don’t see that high fuel economy has got to with that air-fuel mixture. So, when you floor the engine to max power, the engine wants to avoid those high-temperature scenarios in each cylinder and the way the engine does that is by using a really rich air-fuel mixture. In most of the ideal scenario the ratio is set at approximately 14:1 so that the engine uses all that fuel and air so everything goes smooth but when you are running that high boost level your engine will reduce that air to fuel ratio to 11:1 or 12:1 to reduce that high temperature in each cylinder. This might seem strange that injecting more fuel brings the temperature down but as you inject the more fuel the phase change of fuel from liquid to a gas drops that temperature down so you are injecting more fuel solely for bringing that temperature down and removing knock. The downside to that is lower fuel economy as you are using a very rich air-fuel mixture and not creating power very efficiently. The average RPM at which smaller turbos start spooling up is around 1800-2800 RPM (RPM may vary from turbo to turbo) so if you keep your RPM lower than these figures then you will definitely get a very good fuel economy similar to that of a non-turbocharged bigger engine but since the RPM at which the turbo spools up is very very low in smaller engines so it is very difficult to keep your RPM lower than these, and as soon as your RPM goes above this point the turbo will spool up requiring a very rich air-fuel mixture to eliminate knock you will get very less fuel economy so that is why smaller turbocharged engines are not that efficient and get poor fuel economy.
Now the question arises, why do car manufacturers still keep making small turbocharged engines. Well, there are many benefits of making a car turbocharged. You are able to make the engine smaller, subsequently, its weight is less, it has fewer moving parts, lesser friction, less pumping losses. So, depending on the loading scenario or how heavily you drive your car you can get good fuel economy and power as well.
So, the next time you are looking for a new 660cc car because of good fuel economy and think that why not get a turbo for more power too, keep that in mind that you can’t get the best of both worlds, so decide wisely.