What is a Turbocharger and How Does it Work?

Frequently Asked Questions about the Cappuccino.

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What is a Turbocharger and How Does it Work?

Postby Adrian Furniss » Tue Jan 14, 2003 15:39

I recently dusted off an old post concerning the turbocharger and, with considerable enhancement, re-posted it below.

Many thanks to reviewers/co-authors Murray Betts and Ian Linden, whose contributions are a significant proportion of the whole.

The post is organised as follows:

  • Introduction
  • Turbocharger Valves
  • How fuelling and ignition are determined
  • Performance Enhancement
I hope you find this informative.

Happy reading
Adrian Furniss
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Postby Adrian Furniss » Tue Jan 14, 2003 15:54

Introduction

The Cappuccino's engine includes a turbocharger (turbo) to give it reasonable performance whilst remaining within the Japanese K-class regulations, which limit (amongst other things) engine size (660cc) and power (64bhp), but not torque.

As with all vehicle turbochargers, the Cappuccino's increases engine's power and torque (compared to an equivalent non-turbocharged engine) by compressing inducted air prior to its entry into the engine's combustion chambers. The Cappuccino's design also includes an intercooler, which further enhances the effect by cooling the compressed inducted air, making it denser and thereby even more volumetrically efficient as the source of oxygen for the combustion of fuel.

The turbocharger's energy source is the engine's exhaust gases. The turbocharger thus robs the engine of some power, but gives back significantly more than it takes.


Turbocharger Valves

The Cappuccino's turbocharger design includes both a "Waste Gate Valve" and an "Air Bypass Valve".

Waste Gate Valve
Function: Limits the turbocharger's maximum compressor boost pressure in accordance with the engine/vehicle design.
Description: A valve, internal to the turbine side of the turbocharger housing, which allows a portion of the EXHAUST GAS to bypass the turbine fins, thus limiting the amount of boost pressure generated by the compressor. The Waste Gate Valve is motivated, via a rod, by the Waste Gate Valve Actuator, which passively monitors boost pressure and opens fully at a pressure of 90 kPa (0.90 kg/cm.cm or 0.88 bar). The reference level is provided by spring pressure.

Air Bypass Valve (ABV)
Function: Prevents the compressor & turbine slowing when the throttle is closed and thus maintains the availability of boost pressure when the throttle is reopened, in particular following a gear change.
Description: A valve, external to the turbocharger housing, which opens/closes a path from the turbocharger's compressor output to its compressor input. When open it minimises the load on the turbocharger's prime mover, allowing it to rotate freely and thereby minimising the time required to spin the compressor & turbine back up to speed. The ABV is actively controlled by the Engine Control Module (brain) in response to the outputs of various sensors.

Both valves are standard on all Cappuccinos - UK and Japan specification. The functions of these two valves are implemented in all turbocharged vehicles, though the detailed engineering may differ. In particular, some designs implement the function of the ABV with a "Dump Valve" which is identical in function, but differs in that it opens the boost directly to atmosphere rather than to the inlet side of the compressor. Some people refer to the ABV as the "Dump Valve". In the absence of a concrete definition, this is not incorrect, and the distinction is somewhat academic.

Some drivers like to hear a "psshhhh" noise from their turbocharger's ABV/Dump Valve. You will tend not to hear this with the standard Cappuccino arrangement, but it is significantly more noticeable with a low-impedance (high performance) air filter. This noise arises from the opening of the ABV as air momentarily rushes through it to equalise the pressure across the compressor side of the turbocharger. Some Cappuccino owners choose to fit an alternative Dump Valve to enhance the effect, often in association with performance enhancements.


How Fuelling and Ignition are Determined

Fuel injected engines require some sort of measurement of speed and load to enable the correct fuelling and ignition timing to be selected. The Cappuccino Engine Management System dates from the early 1990's, and some of the methods used are not what you would expect to find today. Petrol will burn only within a fairly limited range of Air to Fuel ratio (A/F), and emission controls such as three way catalysts require the A/F ratio to be very closely controlled, particularly in the light/medium load and low/mid speed range where the emission certification tests are run. Most modern engines use a Mass Flow Meter to measure air flow; these are often a hot wire or film that is cooled by the incoming air and the electrical power required to maintain the temperature is a direct measure of the mass flow rate.

The Cappuccino however uses a system known as "Speed/Density". Inside the Distributor are toothed wheels and pick-ups which allow the Electronic Control Unit (ECU) to calculate the speed and the angular position of the crankshaft, using an internal clock to measure times. Also there are two critical devices that allow the fuelling to be calibrated; these are the (Inlet) Manifold Absolute Pressure (MAP) Sensor, and the Inlet (Manifold) Air Thermistor (measuring temperature). The air flow rate through the engine is determined by the pressure difference across the cylinders (i.e. between Inlet and Exhaust Manifolds), and the mass flow depends on the density (i.e. the pressure and temperature) of the air.

When the engine is developed, the Calibration Engineers run the engine and adjust the fuelling to the desired value at a large number of speed and load "sites", and by recording the speed, pressure and temperature at these conditions, the ECU is programmed to recognise the calibration value of the fuelling. It is important to understand that with this system the ECU does not "calculate" the required fuelling (other than interpolating between sites, and applying "modifiers" for coolant temperature, acceleration enrichment etc), instead it makes the measurements and then "looks-up" a pre-programmed injector on-time from a table. This is actually one of the shortcomings of the Speed/Density method; if you alter something that materially changes the relationship between the inlet manifold pressure and the air flow rate, the ECU will not recognise the fact and compensate for it. Changes to the air-filter, intercooler etc do not affect it, because they are upstream of the measurement point, but changes downstream e.g. to the exhaust, or camshafts etc, will change the relationship.

The fuel system pump supplies fuel at a pressure that prevents vapour forming in the pipes and provides the necessary flow capability, and the injectors (which are simply switched "taps", they do not pump) are selected to give the maximum (and minimum) fuel flow as required by the engine at the specified fuel system pressure. A pressure regulator maintains the fuel pressure at a constant value above the Inlet Manifold air pressure.

At speeds and loads within the Emission Certification range, the fuelling is "trimmed" around the calibration value by using the Exhaust Gas Oxygen (EGO, or "lambda") sensor, fitted at the entry to the exhaust catalytic converter. This can sense free oxygen in the exhaust. It works by having a special coating on a ceramic body, one side of which sees exhaust gas and the other side sees atmosphere. If there is free oxygen on both sides there is no electrical output, but if it is present on one side only, a voltage is produced (rather like a battery). This "tells" the ECU whether there is free (un-used) oxygen in the exhaust gas, in which case the mixture must be a bit "lean", so the ECU uses this "feedback" to progressively add fuel and thus "richen" the mixture a little. As soon as the free oxygen disappears the voltage disappears, and the ECU starts to lean the mixture again until it returns. This cycle continues in a "closed-loop" to closely control the mixture to the ideal value that enables the catalyst to clean the gas and remove Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (referred to as "NOx"). At higher loads and speed this "closed loop" system is ignored and the fuelling runs at the calibration value ("open loop") in order to optimise power and control knock, exhaust gas temperatures etc.

Normal combustion is a progressive burning process, and it takes a finite time to take place (somewhere in the region of 45degrees of crankshaft rotation). Therefore to get the best output it is necessary to start the burn before the piston reaches the top of its stroke (i.e. Before Top Dead Centre BTDC). The time taken for the burn depends on the speed and load (and things like Exhaust Gas Recirculation EGR), so the ideal ignition timing at each speed/load site is determined by the Calibration Engineers, and is programmed into the ECU as a speed/load "map". This value is also "looked-up" by the ECU to match the actual running condition as measured by the various sensors.

Some conditions require slightly different values for fuelling and ignition, such as during cranking, when the engine is cold or very hot, if the battery voltage is low, or during acceleration or overrun for example. Various other sensors such as throttle position switches, coolant thermister, cranking signals etc. tell the ECU when these conditions are met and the appropriate calibration values are applied.


Performance Enhancement

The first measure usually considered is to raise the boost pressure delivered by the compressor. From the above explanation, the logical way of doing this is to raise the spring force in the actuator, and, in practice, this can be done by changing the actuator for one with an adjustable setting. Then, by fitting a boost gauge, the boost can be set as desired. But there is always a downside, and, in this case, it is the fuelling. At high revs, the Cappuccino 's Engine Control Module (ECM) calculates the fuel from a "look-up" table, or map, which is believed not to extend to exotic boost levels, although the highest value catered for is uncertain. If the engine is operated "off the map", i.e. at higher-than-catered-for boost levels, it will provide insufficient fuel for correct combustion, or, to put it more commonly, a lean mixture. This results in excessive engine temperature, burnt valves and worse. So, to be safe, it is generally held that the maximum prudent increase of boost which may be applied without enhancement to fuelling is 1.0 bar, i.e. about 13.6%; not bad. Exceed at your peril. The ECM will cut fuel delivery when it senses 1606 mm Hg (equates to 1.13 bar on an average day) - you will know if you get that high; it's very noticeable, and dangerous if another car is riding your back bumper - you were warned!

The cost/difficulty of fitting a replacement actuator will not suit everyone, and a cheaper method is to insert a bleed valve into the pipe, which carries a "sample" of the boost pressure to the actuator. The action of the valve is to lower the pressure to the actuator (whilst not appreciably affecting the pressure delivered to the engine). This makes the actuator allow more boost than it otherwise would, before limiting the compressor output by opening the wastegate. Again, the valve is set up with a boost gauge, as above (and no more!). It's a somewhat crude device; its detractors accuse it of creating turbo-lag, and some inconsistency. The principle can be used to better effect by following the advice in the 2-part article at http://www.autospeed.com/cms/article.html?&A=0670 and http://www.autospeed.com/cms/article.html?&A=0685&P=1. This scheme uses two relatively inexpensive pneumatic valves (see http://www.suzuki-cappuccino.com/albumviewer/viewalbum.php?dev=1&a=42) , and the setting up is well explained. It eliminates turbo lag and "waste gate creep" - not explained here - read the article. It works very well in a Cappuccino, and allows you to eliminate low boost caused by a fatigued actuator spring.

EDIT: The articles were no longer available to browse, but have now returned - in case of difficulty, you can PM Ian Linden for a copy.

If you want to go to higher boost, you can fit a device to the electrical system called a Fuel Cut Defencer - it removes the fuel cut mentioned above, and should make the ECM provide extra fuel as well. How accurately it does this is hard to tell. Superchips, for example, apply this modification using a "piggy-back" board in the ECM, with a bleed valve. Suzuki Sport (no UK dealers - import direct from Japan) will provide a replacement ECM (the N1) at higher cost, but the bleed valve is up to you.

You can also fit a higher output turbo (complete with new actuator), bigger fuel injectors and a bigger intercooler. The first demands the others; the standard injectors will limit the fuel flow despite the demand from your modified ECM, and the additional heat caused by more compression of the inlet air needs to be removed. If this is not done, the result is pre-ignition in the cylinders, which is highly detrimental to engine durability. You can also attack that problem by injecting water (at suitably high pressure) into the inlet manifold; it won't cause rust because it's instantly turned to steam and, in so doing, will cool the charge ("latent heat of vapourisation"). It's not a substitute for a bigger intercooler, but might be handy as an adjunct in hot climates.
Last edited by Adrian Furniss on Wed Aug 18, 2004 10:36, edited 4 times in total.
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Postby Robert Austin » Tue Jan 14, 2003 17:24

If anyone wants a complete beginner's guide to how a turbo works, the following webpage is quite good and has a few pictures and diagrams:

http://www.howstuffworks.com/turbo.htm

The site is quite interesting as it tells you how pretty much anything works... turbos, lock picking, nuclear bombs etc !!!


Robert.
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