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With its latest generation of cars, Honda is now adopting the technique of turbo-charging in a big way. For the mainstream models and the "rest of the world" outside the JDM, the new 10G Civic marks the debut of Honda's new generation of turbo-charged engines. The new 10G Honda Civic now brings relatively high-powered turbo-charged engines to the mass market for Honda owners and also nicely gives Honda enthusiasts a new lease of life.

Newcomers to turbocharged engines might get quite a bit of confusions about the numerous complexities of this application. This first of a series of articles is intended to introduce the very basic concepts of turbocharging to Honda owners who are not familiar with the technology. At this point, it is useful to introduce the often used term "NA" which is "Naturally Aspirated". An NA engine is used to mean an engine which simply depends natural means to get air inside the engine. In a way, it's direct opposite is "FI" or "Forced Induction" which is used to refer to an engine which uses an artificial means to force air into the engine. Turbo-charging is one type of Forced Induction (Super-charging is another very popular technique). The difference between NA and FI should (hopefully) be clear at the end of this article.

Credit : Zephyris. Image used under permission as published by the GNU Free Documentation License

The moving GIF above illustrates the operation of the 4-stroke OTTO cycle internal combustion engine, which is the long-form name for the common petrol engine that is used in most popular cars on the roads today. A complete cycle is made up of the 4-strokes determined by the movement of the piston/crankshaft and the GIF helpfully shows a number which corresponds to the stroke number. Each stroke is actually the action of the piston moving completely from bottom to top or from top to bottom. Thus 2 consecutive strokes will equal one complete revolution of the engine and one 4-stroke OTTO cycle is made up of two complete revolutions (turns) of the engine.

In the GIF, the intake side is on the right and the exhaust side on the left. Air flows in through the intake and then out through the exhaust. The intake manifold/air-filter is attached to the intake which means the right side connects to the intake manifold and air-filter. When air is pushed out, it is routed through the exhaust system out towards to rear of the car (this is for ventilation purposes but also serves a sound reduction function as well). So the header and the "cat-back" (i.e. catalytic converter to resonator/middle-box to muffler) is connected to the left side.

In idealized form (i.e. perfect theory versus real life application), the 4 strokes are as below.

Stroke-1 is called the "intake stroke" and this is where the intake valves opens and the piston moves downwards. Air is sucked in via the intake manifold/air-filter into the cylinder. The amount of air is determined by the capacity or volume of the cylinder when the piston is down at its lowest position, just about to change direction to move up again (technically this is called "BDC" or "Bottom Dead Center"). For a given engine, this volume is equal to the capacity of the engine divided by the number of cylinders. E.g. a 1.6l 4-cylinder engine will have 4 cylinders of ~400c.c. each (engine displacements are almost never completely round numbers). During this stroke, the fuel injectors will also be 'fired' (opened) to spray fuel into the cylinder as well. At the end of the stroke, the cylinder will contain air mixed with atomised fuel, also called the air-fuel mixture.

Stroke-2 is the "compression stroke". The intake valve closes and the piston moves upwards. This action compresses the air-fuel mixture from stroke-1. Compression completes when the piston is right at the top of its travel and just about to move back downwards again (technically this is called "TDC" or "Top Dead Center").

Stroke-3 is the "combustion stroke". The spark plug fires and triggers a combustion (explosion) of the compressed air-fuel mixture from the previous 2 strokes. It is this combustion which generates power by the exploding air-fuel mixture pushing the piston downwards with great force.

Stroke-4 is the "exhaust stroke" and this is where the piston moves up with the exhaust valve open. The piston pushes the burned air-fuel mixture through the exhaust valve out to the exhaust system. The 4-stroke cycle completes and repeats with stroke-1 again.

It is important to understand that this description of the 4-stroke cycle is "ideal" or completely "in theory". In actual practise, a lot of things don't happen instantaneously. E.g. in Stroke-3 "combustion", when the spark plug fires, the air-fuel mixture doesn't instantaneously explode uniformly. Rather the part of the air-fuel mixture right at the spark plug will start to burn first as a response to the spark and the explosion will spread outwards from there. This means that maximum pressure/force generated by the exploding air-fuel mixture in stroke-3 is not instantaneous. It is still inside a very short period of time, but when an engine is spinning at 6,000rpm for e.g. this is 100 revolutions per second which means one stroke takes around 20 milliseconds (0.02 seconds) and the delay becomes significant. So in real-life, the spark-plug timing is 'advanced', i.e. brought forward to a point somewhere at the end of stroke-2 "compression stroke". The idea is that by the time stroke-2 completes, the exploding air-fuel mixture will be at a point where it will push downwards against the now downward moving piston most optimally.

However, for the purpose of this article and for best understanding of the principles of turbo-charging, it is best to think of the whole process in its perfect, idealised theory form.

It's all about the Power

In an engine, it is the fuel/petrol which is burned and exploded in stroke-3 which generates the engine power. Technically, how strong the explosion is determines how much force the piston is pushed down and this determines the torque generated by the engine. This then directly determines the amount of power the engine is delivering at the specific rpm at which the torque figure applies to. Fuel needs oxygen to burn and air provides the oxygen. How much torque (and power) is generated is determined by how much fuel is burned. But how much fuel can be burned depends directly on how much air is available in the cylinder. Fuel has a very specific combustion property and the fuel we use on our (street) cars burns most optimumly (i.e. most completely) at an air-fuel mixture of 14.7 and generates highest torque at a mixture of around 13.3 (for NA or non-turbocharged engines).

What this means is that if we want maximum torque (and power) in stroke-3, we need more fuel and this means during stroke-1 we need to get more air into the cylinder. However, physically the maximum amount of air that can go into the cylinder is fixed, limited by the volume/capacity of the cylinder. One way of getting more power (but not torque) from an NA/non-turbocharged engine is by spinning the engine faster (higher rpm). Thus, the legendary Honda B-Series and K-Series engines of the last decade will spin up to 8,000rpm or more, compared to 6000rpm+ of a standard engine which allows them to generate very high amount of power compared to their size (displacement). A racing car engine will run at even higher rpm (e.g. 20,000rpm in the Formula-1 cars of a decade ago). This allows a small-ish engine to generate phenomenal amount of power (900-1,000hp for the case of those F1 cars) but the engine torque is still limited to the displacement of the engine and the number of cylinders.

So for NA engines with its greatly lower rpm, if we want more power it means we need more torque and outside of Type-R engines, the only way is to increase the displacement of the engine, so that each cylinder is larger in volume (or there are more cylinders) which allows more air to be ingested and thus more fuel to be burned.

By Anonymous -, CC BY-SA 4.0,

The moving GIF above shows one method of getting more air into the cylinders during stroke-1 so that more torque and thus more power can be generated by an engine of a given size/displacement. This method is called "turbo-charging".

The idea/principle of turbo-charging is to overcome the physical limitation imposed in stroke-1 and to get more air into the cylinder than what its physical volume allows. An 'air-pump', called a(n) (air) compressor is used to push air at high speed into the cylinder in stroke-1, instead of allowing air to flow naturally into it. In a way, it can be thought of as "force-feeding" air into the cylinder. The process compresses the air inside the cylinder so the pressure (before stroke-2 compression) is higher than atmospheric pressure. Thus, in turbocharging we talk about the initial air being in a certain amount of 'boost', i.e. higher than atmospheric pressure. By having denser air in the cylinder, we have more oxygen content (eventhough the physical volume is still the same as the cylinder). This allows more fuel to be burned/exploded in stroke-3 and thus higher torque and more power.

The compressor can be driven by various methods and in turbocharing, it is driven by the 'turbine' which is like an "air-wheel". The exhaust gas which is expelled in stroke-4 is used to drive the turbine. And the turbine is connected to the compressor by a shaft. So turbocharging is effectively using unwanted exhaust gas, which needs to be dumped out of the engine anyway, to indirectly drive an air-compressor via a turbine so that more air is pushed into the engine during the intake stroke. The mechanical device of the connected turbine and compressor is called a "turbo-charger". Turbo-charging allows the engine to generate more torque and with it, more power.

In addition to the turbo-charger, the moving GIF above also shows the addition of an "Intercooler", labelled "Charger Air Cooler" put in between the compressor and the engine. The principle of the Intercooler is to cool the air coming from the compressor of the turbo-charger before it goes into the cylinder in stroke-1.

The turbo-charger itself runs at a very high temperature in operation. Firstly as the turbine is spun by exhaust gas, this heats up the turbo-charger greatly. In addition, the process of compressing the air also increases the ambient temperature of the air. The Intercooler is placed in an area of high air-flow in the engine bay, usually right at the front of the car (the grill area) in front of both the radiator and air-cond coil. The air force flowing through the Intercooler will cool the compressed air down before it reaches the engine.

The next article will look at some of the problems associated with using a turbo-charger to increase the torque/power output of an engine. A bit of it is already covered with the Intercooler above and a bit more will be looked at next.

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June 2016
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