02 Dec 00

Manifold Pressure, Boost, Knock Sensors, Octane Rating and Anti Knock Index (AKI)

After reading some of the posts which touch on the above topics I believe that I can add some information and clarification on them and their relationship to each other and engine performance and engine longevity. I am assuming that everyone has a basic understanding of the four stroke (Otto) cycle internal combustion engine.

Fuel, Octane Rating and AKI

Gasoline is a very complex hydro-carbon substance, with many additives, that must do many things. One of the more important additives is the one which increases the octane number of the fuel. Ordinary gasoline, as it comes out of the refinery cracking tower, has a quite low octane rating. The octane rating of a gasoline is defined, very simply, as the gasoline's resistance to knocking or pre-ignition; the higher the octane number the higher the resistance. Knocking or pre-ignition is defined as the spontaneous ignition of the mixture in the cylinder by means other and/or in addition to the spark plug; spontaneous ignition ocurrs when the temperature in the mixture in the cylinder gets above 400 to 500 degrees Celsius. Note that octane rating has virtually nothing to do with most other charcacteristics of the fuel, although some of the additives used today to increase octane rating do increase the mass density of the fuel slightly. I will not go into what is added to fuel to increase octane number except that it has been made much more difficult and costly since the banning of tetra-ethyl lead compounds and their scavenging agents from auto fuel. Many years ago there was an aviation gasoline that had an octane number of 115/145; the two numbers corresponding to lean and rich mixtures in the aircraft engine using the fuel, and which are roughly analagous to MON and RON described below. Some aviation gasoline today still has some tetra-ethyl lead in it, ie avgas 100LL, the LL standing for Low Lead. Many piston engine aircraft today srtill have manual mixture controls, which the pilot uses to adjust the mixture to the performance requirement of the engine. The "antique" choke was a mixture control of sorts; it richened the mixture when cold, at starting, so the spark plugs were able to ignite the mixture.

The octane number of a gasoline is determined in a laboratory test engine. A mixture of two hydro-carbon compounds, iso-octane and n-heptane are used as a test fuel. Iso-octane, from which the name of the octane rating is derived, has very good anti-knock properties and has been assigned an octane rating of 100. The hydro-carbon n-heptane has a very low anti-knock properties and has been assigned an octane rating of zero. Different mixtures of iso-octane and n-heptane are burned in the test engine in the laboratory until the knocking charcateristics are the same as the fuel being tested. The fuel is then assigned the octane number which is the percentage, by volume, of iso-octane in the iso-octane n-heptane mixture. It really is much simpler than it sounds. However, there are two different standards by which the octane number is calculated; they are called the RON and the MON for Research Octane Number and Motor Octane Number. I will not go into their differences or specifics here except to say that RON refers to engine operation at lower rpm's and higher manifold pressures and that MON addresses engine operation at higher rpm's, high cylinder/mixture temperatures and variable ignition timing and that most car companies, including Volvo, and most major gasoline companies average them, ie add them together and divide by two, which Volvo and some fuel companies now call the AKI, Anti-Knock-Index, when stating the octane rating of the fuel. Incidentally, Volvo has had this simple formula for the computation of AKI, incorrectly printed in all their operator manuals for many years now; I just checked my '01 manual and it is exactly the same as it is in the '94 manual. Oh well, I guess that they figure no one reads the manuals so why bother to check them!

Knocking or pre-ignition has to do with the type of combustion and the type of initiation of that combustion in the cylinder.

In a normally operating engine the fuel in the cylinder is ignited by the spark plug and burns, this burning taking place over much of the power stroke of the engine and we say that the flame front, or edge of burning, travels at a certain, relatively, high speed. The pressure in the cylinder, due to the combustion is maintained at a design value for much of the travel of the piston during its power stroke. Also, equally important, the temperature rise is such that the expansion due to piston movement and the engine cooling system is able to keep it from reaching damaging levels.

Please note and ponder here, that when we are talking about speed, 5000 rpm, is at the high end of the Volvo engine performance band; it is about 83 revolutions per second; this means that each piston starts and stops 166 time in that second, and that the cycle of intake, compression, power and exhaust have occurred more than 41 times! Just contemplate what is going on in that cylinder, and with the computer control system, while the second hand on your watch clicks ahead one division. (If I sound like I am in awe of this, I plead guilty.)

If the engine is not operating normally, ie. knocking or pre-ignition is occurring, the ignition of the fuel is occurring too early, perhaps much too early, and not only by the spark plug, but by the very high temperature of the fuel/air mixture in the cylinder, because of too high compression ratio, too high manifold pressure or too high operating temperature or too low octane fuel or some combination of all four. Most importantly, the combustion process is explosive,or nearly so. Because of the abnormally high temperature the combustion process takes place almost simultaneously throughout the cyclinder; the flame front travel speeds are very much higher, ie. an order of magnitude higher and approaching the speed of sound, than the normal burning process flame front speeds and most important of all the cylinder pressure spikes to a very high value as does the temperature. It is possible in some situations for the simultaneous ignition of most of the mixture in the cylinder after spark ignition has taken place and normal combustion has started. The extremely quick high pressure build up is what gives the knocking sound and the high pressure and temperature can very quickly damage pistons and valves by both mechanical and thermal means; engine performance will drop markedly at the onset of pre-ignition. (The pre-igntion can also be caused by hot spots in the cylinder, on the head or top of the piston, normally glowing carbon deposits. Normally these engines have many other maintainence problems too, and this will not be discussed here.)

The compression ratio of an engine is simple the number of times smaller the cylinder volume is when the piston is at top dead center than it is when the piston is at bottom dead center; or it is another way of saying how much the mixture is compressed before it is burned. Otto cycle engines can run with compression ratios as low as 4 or 5 and as high as 12 to 14 with exotic anti-knock fuels. Normally, with turbo-charging, compression ratios are kept under 9. In general, higher compression ratios mean higher fuel efficiency and vice-versa. Higher compression ratios mean higher mixture temperatures and pressure in the cylinder before combustion takes place; as long as pre-ignition does take place in all or most engine operating regimes, higher compression is better.

Knock Sensors

Knock sensors are neat little piezo-electric devices, ie. they develop a voltage when vibrated or stressed, and when combined with some fairly complex filtering, computer analysis and data comparison they produce a signal that indicates knocking or pre-ignition is occurring and which cylinder(s) it is occurring in. They are able to do this inspite of all the other vibration, etc.,that is going on at the same time. The knock sensors each produce a base signal when they and the engine are operating normally which the ECU knows. When pre-ignition occurs, the knock sensor signal changes, is recognized by the ECU and then the ECU retards the ignition timing and richens the mixture; if knocking does not stop after a certain pre-determined time then the ECU will reduce the manifold pressure by signalling the turbo charger control valve to do its work on the waste-gate actuator. There are at least three DTC's relating to KS stuff that I know of: 1-4-3 and 4-3-3 which relate to front and rear KS respectively and 1-1-2 which relates to RAM where KS data is stored and which will not be discussd here. The occurrence of either or both of the first two can mean a number of possible things, from bad KS's to bad circuitry to a misinstalled camshaft drive belt. However, the setting of either DTC is serious business; knock control is inhibited, hence manifold pressure is limited, ignition timing is retarded and the mixture is richened. In short, you using more fuel and getting much less power.

A beautiful description of all this and much, much more, mostly in not very technical language, is presented in Volvo Manual TP 2301202 which describes the Motronic 4.3 system. This is a very good manual to have. (I would also suggest that if you have a newer Volvo for which there are no printed manuals it might be worth your while to buy some of the manuals for the nearest car to yours since many things have not changed that much.) I am a manual freak. I have always had a complete set of manuals for all my Volvos going back to the 1966 122s. The complete set for that car is about the same thickness as the manual mentioned above. One other very useful book is the Bosch, Automotive Handbook, 4th Edition, 1996. There is probably a later edition available now. You can get it from SAE or Amazon, ISBN 1-56091-918-3. The newer edition will have a different ISBN.

Manifold Pressure and Boost.

The engine is an air pump of large proportions. In the ideal, complete combustion of one kilogram of fuel, about 14.5 kilograms of air are required; a lean mixture requires more air and a rich mixture requires somewhat less air than ideal. The closer we are to this ideal ratio of fuel to air the cleaner the engine combustion process is and the easier it is for the catalytic converter and oxygen sensors to do their work. We are not trying to talk about emissions here so no more about this. The above manual covers it very well.

In general, leaner mixtures burn hotter and are more economical of fuel. For rich mixtures the reverse is true. Power is the rate of doing work. To produce more power we must burn more fuel per unit time. To burn more fuel in the cylinder we require more air, really more oxygen which is a component of the air.

The air is distributed to the cylinders by the intake manifold, which has a throttle valve at its entrance to control the flow of air; when the throttle is nearly closed, not much air gets past it and we say that there is a large pressure drop across the throttle or that the manifold pressure is low. When the throttle is wide open, the pressure drop across it is very small and the manifold pressure is high, essentially the same as atmospheric pressure where the automobile is situated. Normal atmospheric pressure at sea level and standard temperature is about 14.7 pounds per square inch, or 29.92 inches of mercury, or 760 mm of mercury or 1017 millibars, or one atmosphere; take your pick, I will use atmospheres in this discussion. So the maximum amount of air which will flow into the engine when the throttle is wide open determines how much fuel we can burn per unit time and it determines the maximum power output of the engine. We say that this manifold pressure is equal to one atmosphere. We also call this type of engine a "normally aspirated" engine or an "unboosted" engine; the only means of pushing air into this type of engine is the natural pressure of the atmosphere.

If we put an air pump upstream of the throttle valve, we can blow more air into the engine, burn more fuel and produce more power. We call this pump a supercharger; they may be driven by mechanical means such as gears or belts, by fluid couplings, or by exhaust gases in which case they are now almost universally called turbo-chargers. All the more powerful Volvo gasoline engines are turbo-charged, which simply means that they are capable of operating at manifold pressures which are higher than atmospheric. Sometimes we call intake manifold pressure operation above atmospheric, "boosted" pressure, or for short just, "boost". This term is taken from the aircraft industry. The real beauty of this is that there is much thermal energy in the exhaust gas of the Otto Cycle gasoline engine which normally goes out the tailpipe; with the turbo-charger some of this energy is used to spin the turbine which drives the air pump or compressor to push extra air into the intake manifold; it is almost, but not quite, like getting something for nothing!

There are many things which must be taken into account when designing a small, high output turbo-charged engine, but we do not need to go into them here. However, to keep size and power output in perspective just note this; the Cadillac Northstar engine is normally aspirated, displaces 4.6 litres and in the DTS sedan is rated at 300 HP, ie. it is about twice the displacement of the Volvo B5234T3, but only produces 53 more HP!

In piston engine driven aircraft we have a graduated scale manifold pressure gauge so we can always monitor exactly, with the tachometer, the engine oputput; in older Volvos we used to have a manifold pressure gauge of sorts but it only had one mark on it and two or three ranges; the one mark in the center being exactly one atmosphere, or that manifold pressure which corresponds to wide open throttle if there is no pressure charging. When the engine is stopped there are no flow losses so the needle sits at the mark as well. When the engine is running at a high power setting, the needle will reside, normally to the right of the mark in the white range on newer cars and yellow range on older cars. The farther that the needle is into the white or coloured range the higher the boost. Older cars had a red or orange range beyond the yellow range to indicate even higher or dangerous manifold pressure. For all operation where the manifold pressure is less than atmospheric the needle lies to the left of the mark in the blank or unmarked area of the gauge; with closed throttle, at higher rpm's, it moves almost to the left side of the blank range.

For those of you who have never looked at a turbo-charger, it is really a very simple device, but one that operates in extreme conditions. Two small turbines, approximately 2 inches in diameter, one on each end of a very short shaft. One turbine is the compressor or pump turbine and it operates in an aluminum scroll case. The other turbine drives the shaft and it operates in a cast iron scroll case. The structure between the scroll case supports the fully floating bearing for the shaft as well as supporting the scroll cases. The bearing has a very large oil pipe supplying pressurized oil for lubrication, support and some cooling; its jacket has a cooling fluid connection to provide additional pressurized cooling from the engine cooling system. Remember that the exhaust gas temperature is in the order of 1000deg C. Also in the exhaust gas supply scroll case is a small control valve which can dump some of the exhaust gas to bypass the drive turbine; it is called a waste gate and it is activated by a waste gate actuator which is controlled by the turbo-charger control valve which, of course, operates under the auspices of the ECU. If the waste gate is opened, then less exhaust gas goes to the turbine, it slows down, and the output pressure from the compressor turbine is reduced. Neglected in this discussion is all the hot and cold plumbing to connect the turbo charger to manifolds, etc.

Once the engine is running, the turbo-charger immediately starts turning, but slowly and the manifold pressure is not affected in any measurable way. As engine power is increased, exhaust gas production increases and turbine/compressor speed increases as does the output air pressure; the maximum turbine speed can exceed 100k rpm's in some small turbo-chargers when the engine is operating at high power settings and the output pressure produce can easily exceed one atmosphere above local atmospheric pressure. There is a finite, measurable length of time required to increase turbine rpm after power is iincreased and before which more air at higher pressure is delivered. This is the so called turbo lag. It is reduced by making very small rotating turbine elements which have a very low rotational inertia and thus accelerate quickly; because they are so small is the reason that they have to rotate so quickly.

In the '01 V70 T5 the maximum boost for the B5234T3 is 0.90 atmospheres or 13.3psi; normal boost is about 0.80 atmospheres or 11.7 psi. This data from TP 0309201. For comparison, the maximum boost for my '94 850 is about 0.70 atmospheres or 10.4 psi. TP 2301202.

The magnitude of the manifold pressure must be available at all times for the ECU to do its job. It is not sensed as a pressure, which would require other data for corrections, but as the total mass of air entering the intake manifold. It is always the total mass of air which determines the amount of fuel which can be burned. The Mass Air Flow, MAF, sensor is located with its work surface, in the intake air flow, upstream of the throttle and just downstream of the air filter. It is essentially a hot film anemometer that operates at a high enough temperature, 179 deg C, so that it is self-cleaning. Remember that 14.7 kgms of air are measured for every kgm of fuel that is burned. The ECU, with the air mass quantity and with consideration of all the above and much other data not discussed here, continuously computes how much fuel each injector should provide each cylinder and when this fuel should be injected and when the spark plugs should fire. You can see that it is quite a formidable task. Cars are so clean burning today that any poor soul who tries to end it all in a closed gargage with his new car engine running will probably starve to death or die of boredom, before carbon monoxide poisoning occurs.

So how does all this work together? Engine power output is an exponential function of rpm's and is nearly directly proportional to manifold pressure; commence increasing either one or both and power output starts to increase; the converse is also true. High manifold pressures at low rpm's induce higher temperatures and pressures in the cylinder, with the potential for some of the above bad consequences. If you are looking for increased power it is normally quicker to use the transmission, ie., increase rpm's; in general, that type of driving behaviour will increase engine longevity. If you want a rule: "TO INCREASE POWER INCREASE RPM'S FIRST THEN INCREASE MANIFOLD PRESSURE."

Summary

Given all the forgoing, what are the lessons for the average Volvo driver in a turbo-charged engine car?

1. Use premium, high octane fuel.

2. Use clean filters, air and fuel.

3. Use high quality, correct spark plugs. Volvo specified plugs last about half as long as Volvo says!

4. Do all engine (and other) maintenance routinely; it is so cheap compared to repairs.

5. Monitor fuel consumption; it is a great indicator of engine health and your driving habits.

6. Keep a written log, date, time, price, etc. of all your car maintenance and activities.

Finally, unless you have a very good understanding the Bosch/Volvo engine control system as applied to Volvo engines I would not mess with it. Volvo cars are very fast but you are always going to have serious trouble with a stock ,five litre American V8, especially if it is hooked to a well maintained torque converter transmission. Take consolation in that you could probably win the race with it from 160 to 200 kms per hour.

I have driven well over a million kms in many Volvos over the last 35 years. For the most part, I do my own maintenance. We have two Volvos now, a '94 850 Turbo with 205k kms and a '01 V70 T5 with 10k kms. The T5, in three months, has been absolutely trouble free. The '94 has been almost a perfect car, except for air conditioning. It still starts, runs, and idles the way it did when new. Fuel consumption kept improving for the first 100k kms or so and has been the same ever since. It is quieter than the T5. About a month ago the first check engine light came on; front KS signal bad. I have the KS repair kit and the soon as the light comes back on I will change both knock sensors. My sons drive six more Volvos of vintage '83 to '98.

All the best.

Robert A. Froebel