This article was last published in Issue 42 (June 2017). It has since been updated for Paul’s book ‘Classic Engines, Modern Fuel’ and what you are about to read is Chapter 4 of Paul’s book. I have been granted permission to reproduce Chapter 4 by Rod Grainger of Veloce Publishing Limited to whom I am most grateful.
Search on the internet for information on how petrol engines work and you will find the answer: “there are four cycles, induction, compression, ignition and exhaust”. Or as described in this chapter, suck, squeeze,bang and blow. However, the operation of these engines is more complex than this simple description. To help the reader appreciate the results of the Manchester tests, this chapter introduces some of the concepts affecting the combustion of fuel in a four-stroke spark-ignition engine. It describes the journey taken by a single cylinder in an engine running at 3000rpm. While the valve timings apply to an XPAG, they are virtually identical to any petrol engine, old or new.
Our piston completes the four stages of the cycle in 40 thousandth of a second (40ms). Think how fast one second is and imagine that 1ms was the same as one second. On that timescale, one minute would last 17 hours! It would take around 11 hours for our cycle to complete. 1ms is so fast that even gasses act like solids.
The start of our journey is when the piston is at Top Dead Centre (TDC). At this point you might expect the exhaust valve to snap shut followed by the inlet valve opening as quickly. Valves cannot open and close instantaneously. Delays in opening the inlet valve reduces an engine’s power. The engineers who designed these engines knew valves could start to open earlier or close later than expected.
At the start of our journey, the inlet valve will already have begun to open. It started to open 0.6ms (11o before TDC). The exhaust valve is still open. It will take another 1.3ms (24o after TDC) before it closes.The 1.9ms when both valves are open is called valve overlap. This is beneficial at higher rpm.
At the top of the ‘blow’ stroke, the piston has expelled most of the exhaust gasses. As the inlet valve starts to open, a “scavenge” effect takes place. The rush of gasses into the exhaust port draws in air/petrol mixture through the inlet valve.
At TDC the cylinder is not empty. The 45.5cc combustion chamber (about 15 per cent of the 312.5cc cylinder volume) still contains 1200oC exhaust gasses from the previous cycle. As the piston starts the ‘suck’ stroke, these will continue to vent through the exhaust valve until it closes 24 o after TDC. The remaining hot gasses will cool as they expand. If you ever studied Physics, you may remember Boyle’s Law: as a gas expands, it cools, and when compressed it gets hotter.
As the piston falls it will reach the point where the pressure in the cylinder becomes lower than the inlet manifold pressure. The air/fuel mixture will start to flow into the cylinder. Induction has begun.
The volume of mixture entering the cylinder is controlled by the throttle butterfly. This is a brass disc that pivots when the throttle is pressed. As it rotates, it reduces the area of the restriction in the inlet manifold allowing more mixture to flow into the engine. As more air flows through the carburettor, the suction piston responds. It moves upwards withdrawing the tapered needle from the jet. This allows more fuel into the air stream. The way the SU or variable jet carburettor works is described in more detail in Chapter 5.
To get the greatest power from an engine, the suck cycle needs to induct as much air/fuel mixture as possible. In a normally-aspirated engine, the volume of mixture entering the cylinder depends on engine capacity. As petrol vapour occupies about 14 times the volume of liquid petrol, the more liquid petrol that can be inducted, the greater the power output.
Superchargers or turbochargers increase the pressure in the inlet manifold, forcing more mixture into the cylinder. This is why these engines generate more power than normally-aspirated engines with the same capacity.
Depending on throttle setting and engine rpm, around 10% of the petrol will evaporate in the carburettor, cooling it. The remaining 90% will enter the engine as different sized droplets of liquid petrol.
In normal road use, when the engine is running at part throttle, the volume of petrol evaporating in the carburettor will not be noticeable. However, for those who want to maximise power output, it reduces the overall volume of petrol entering the engine and hence its power. Additionally, the cooling effect can also cause the carburettors to ice up, especially on cold, damp days. This is more likely in engines with exposed carburettors such as motorbikes.
As discussed in Chapter 2, modern petrol has more front endcomponents than classic petrol. These evaporate at lower temperatures. This has two negative effects on carburetted engines. Firstly, it increases the volume of petrol evaporating in the carburettor. Less liquid petrol is inducted, reducing power output. Secondly, it increases the cooling of the carburettors, increasing the risk of icing.
The first air/fuel mixture entering the cylinder meets the residual hot exhaust gasses. These heat the incoming mixture evaporating some of the petrol droplets and cooling the residual gasses in the process. Even though these are extremely hot, they will not contain enough energy to evaporate all the inducted petrol.
The inlet valve on many two-valve per cylinder engines is offset to the side of the cylinder. The advantage of this is that it causes the inducted mixture to swirl during the suckstroke. This both helps the petrol droplets disperse in the air and increases turbulence, something that is very important during the ‘bang’ cycle.
Before petrol can burn, it must be a vapour. To get the optimal mixture for the bang cycle, all the liquid entering the cylinder must evaporate or boil and mix with the air. This boiling is unlike that in a kitchen kettle used for water. In a kettle, bubbles form in the bulk of the liquid. Because the suck cycle is so fast, the petrol in the cylinder can only evaporate molecule by molecule from the surface of the droplets. The small droplets with a large surface area relative to their volume, evaporate the fastest.
After reaching bottom dead centre (BDC), the piston starts to rise. Induction continues for another 3.2ms (57O after BCD) until the inlet valve closes. During this time the air/fuel mixture entering the cylinder some 90mm above the piston, does not feel the effect of its upward motion. The mixture continues to flow into the cylinder. This increases the cylinder pressure about 0.2 to 0.5lb/in2 (140 to 350kg/m2) above that of the inlet manifold. This is called the stagnation pressure.
The ‘squeeze stroke does not start in earnest until the inlet valve has closed. It continues for another 6.8ms until the piston reaches TDC, after 20ms or halfway into our journey.
During this stroke, the pressure and temperature of the mixture increases. This provides extra heat to evaporate more liquid petrol.
Pockets of rich and weak mixture will form as the liquid petrol evaporates. Turbulence in the gases disperses these pockets as the squeeze stroke progresses.
In common with many modern designs, the combustion chamber in the cylinder head of the XPAG is boat shaped. As a result, the outside edges of the cylinder head overlap the bore.
As the piston approaches the top of the stroke, the gasses on the outside edge are ‘squished’ into the combustion chamber, increasing turbulence and mixing.
At the end of the compression stroke, liquid petrol may still be present in the cylinder. This can be trapped between the piston and cylinder wall, around the valves, or as large droplets of fuel that have not evaporated. Even if the carburettor is delivering the correct mixture, the presence of liquid petrol results in a weak mixture at the end of the stroke.
There are two ways the vapour of a flammable liquid can ignite. The flash point is the lowest temperature at which an ignition source, such as the spark plug, can ignite the mixture. The lowest temperature at which it will spontaneously ignite, burning without a source of ignition, is called the autoignition temperature.
Autoignition is very bad for an engine. It causes the pressure in the cylinder to rapidly increase, resulting in pinking or knocking. This is a mechanical tinkling sound that occurs typically at full throttle and low rpm. It sounds like pebbles being shaken about in an empty tin. An ideal fuel has a low flash point and high autoignition temperature. The higher the octane rating of the fuel, the higher the autoignition temperature.
The XPAG, in common with many older engines, has a low compression ratio of 7.25:1 compared with 9:1 or higher in modern engines.
In the 1950s, improved petrol quality allowed compression ratios to be increased. With higher compression ratios there is more heating of the air/fuel mixture during the squeeze stroke, and more liquid petrol will evaporate. Its final volume is smaller, allowing it to burn more efficiently. These engines produce more power than lower compression engines of the same capacity. The main disadvantage is that the engine is more prone to pinking or knocking caused by the mixture autoigniting during the compression stroke.
Compression is initiated by an electrical spark in the gap of the sparking plug.
After the sparkplug fires, combustion takes place in three phases:
- A fireball of burning mixture, about the diameter of a human hair, is created between the electrodes of the sparkplug. On the timescale of the running engine, this flame front expands exceptionally slowly.
- Once the fireball has grown to about the size of the sparkplug electrode, turbulence takes over. This spreads the ignition points, causing the remaining mixture to burn very rapidly.
- As the temperature in the cylinder rises, any remaining liquid petrol is vaporised. These temperatures are sufficiently high to burn any hydrocarbons in the cylinder, including oil that may have leaked past the valves or piston rings.
As petrol burns, the hydrocarbon chains break down; the hydrogen (H) combines with the oxygen (O2) in the air to produce water (H2O). The carbon (C) combines with the O2 to produce carbon dioxide (CO2). Each litre of petrol liberates a huge 33 million joules of heat energy – enough to boil 100 kettles of water. The heat energy from the burning fuel increases the temperature of the gasses to over 2000oC.
The theory Boyle’s Law states that as the temperature of a fixed volume of gas increases so does its pressure. As the pressure increases, it forces the piston down the cylinder. This is how the engine converts the heat energy in the petrol into power.
As the piston falls so does the temperature and pressure of the gasses. Greatest power is produced when the pressure in the cylinder reaches its peak at 17 o or 0.9ms after TDC.
After the spark plug has fired, it takes approximately 2.6ms for the mixture to burn and reach peak pressure. To provide enough time for this to happen, the sparkplug needs to be fired before the piston reaches the top of its stroke. This is called ignition advance. Simplistically, the time to reach peak pressure is constant and independent of rpm. Hence, as rpm increases, and the piston is moving faster, the sparkplug needs to be fired earlier in the cycle to give the same time for the fuel to burn. A graph showing the ignition advance against rpm is called the advance curve.
Typically, at 3000rpm, the sparkplug is fired 30o before the piston has reached TDC. A race begins. As the flame front grows and the pressure in the cylinder rises, it is working against the piston which is still moving upwards. At 30o advanced, the piston still has 8.5 mm to travel (9.3% of the stroke) before it reaches TDC. Knocking or pinking causes rapid increases in cylinder pressure before the piston reaches TDC. This puts an excessive load on the piston and big end bearing causing damage.
On 1930s and earlier cars ignition advance was set manually, typically by a lever on the steering wheel. On later cars this is done automatically by bob weights in the distributor. These fly out as engine rpm increases.
The growth of the initial fireball also depends on the pressure of the air/fuel mixture in the cylinder. This is mainly dependent on throttle setting, not, as may be expected, compression ratio. At light throttle settings, cylinder pressure is low and the growth of the flame front slower. To give more time for the air/fuel to burn, it is necessary to further advance the ignition timing. On later cars this is done by a vacuum pod on the distributor. This is connected to the inlet manifold to measure its pressure. This, in turn, is a measure of throttle setting. Earlier cars do not have a vacuum advance.
Maintaining the correct ignition advance is important. Running an engine too advanced will result in pinking or knocking and damage to the piston and big ends. Running too retarded increases the exhaust temperature, resulting in burned pistons and exhaust valves and damage to the cylinder head.
The ‘bang’ stroke suffers from a problem called ‘cyclic variability’. The time taken for the air/petrol mixture to burn critically depends on a number of factors.
These include the air-to-fuel ratio (AFR) around the sparkplug, the level of turbulence in the gasses, etc. Even with a precisely timed spark, minor differences in these factors cause the timing of the peak pressure to vary on each cycle. This is discussed in Chapter 6.
Unfortunately, the power stroke ends all too quickly, 7.1ms after TDC when the exhaust valve starts to open (52o before BDC). The high pressure gasses rush out of the cylinder in a process called ‘blowdown’. Blowdown utilises the remaining combustion pressure to “get the gas in the exhaust moving”. Without this effect, energy would be lost during the exhaust stroke as the piston would have to push the gasses out of the cylinder.
Each piston is powering the car forward for just 18% of the time of each cycle.
If the combustion were perfect, a mixture of water vapour, carbon dioxide and nitrogen rushes into the exhaust, expanding and cooling to around 500oC. The piston reaches BDC, 2.9ms after the exhaust valve started to open. As it rises, the remaining exhaust gasses are pushed out of the cylinder.
Unfortunately, combustion is not perfect; petrol vapour requires oxygen to burn. Droplets of petrol that vaporise late in the cycle leave pockets of poorly mixed vapour. Although the temperature is sufficiently high to cause these to burn, the absence of oxygen means they will not burn properly. They result in unburned hydrocarbons or carbon monoxide in the exhaust gasses. As the unburned hydrocarbons travel down the exhaust system they may mix with oxygen. As they burn, they further increase exhaust temperatures.
A gas analyser reveals a great deal about the combustion process. The unburned hydrocarbons show how much petrol has been unable to burn in the cylinder or exhaust due to a lack of oxygen. This can arise either because of a rich mixture or poor combustion as described above. NOX or nitrous oxide, (NO) is produced at high combustion temperatures when the nitrogen in the air oxidises. NOX is bad for three reasons : it uses energy, reducing the engine’s efficiency; it reduces the amount of oxygen available for the fuel to burn; and it’s an atmospheric pollutant. The presence of NOX is usually an indication of high ignition temperatures caused by a weak mixture.
In contrast, high levels of unburned hydrocarbons or carbon monoxide indicate insufficient oxygen. This is either due to a rich mixture or poor combustion.
What about modern cars? While the valve and ignition timing will differ slightly from an XPAG, the journey described above is very similar. There are three main differences:
- The fuel is injected as very small and evenly sized droplets, typically 50μm diameter (about the size of a human hair), a fifth of the size of those produced by a carburettor. These not only mix with the air more effectively, caused in part by the careful design of the inlet manifold, they evaporate faster too. Ultimately, this creates a more evenly distributed mixture of air and petrol vapour in the cylinder before the ignition fires.
- Compression ratios are higher than in classic cars, increasing the compressive heating. There is more energy to vaporise the petrol.
- The ignition timing is continuously adjusted to ensure that the mixture burns optimally. These engines are typically far less advanced than the XPAG. As a result, the race between the piston and flame front is shorter and no longer left to chance.
At 3000rpm, our journey ends after 40ms only to start over again. Each cylinder completes the cycle described above 25 times per second. As you can see from this brief description, it is far more complicated than just induction, compression, combustion and exhaust.
The link to buy Paul’s book Classic Engines, Modern Fuel is: https://classicenginesmodernfuel.org.uk/Clubs/ModernFuel/Book.aspx?dyn_menu_mainmenu=1000001
Paul is donating all the royalties from the book to helping children with their schooling in Tanzania. Issue 65 (April 2021) of TTT 2 contained an article about this.
I’m absolutely delighted to say that the book has been an outstanding success, with copies sold all around the world.
The book’s 152 pages are divided up into the following chapters:
- The XPAG tests
- Petrol volatility
- Ethanol blended petrol
- Suck, squeeze, bang & blow
- Combustion and cyclic variability
- weak mixture
- Results from the tests – slow combustion
- Keep the fuel system cool
- Choice of fuel
- Tuning carburettors
- Tuning the ignition system
- Fitting a vacuum advance
- Testing an engine’s efficiency
- Appendix: tuning SU carburettors
Paul has owned his TC since 1967. He bought it for £60 from his mathematics teacher in school. His father was pretty dismissive about the car that Paul was going to buy, saying that it was a load of rubbish! However, Paul was supported by his mother and the sale went ahead.
The car as bought, back in 1967.
The car as it is now – pic taken in 2020. It has been rebuilt twice.