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When Things Go Bang

 by james on 20 Oct 2015 |
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When Things Go Bang (In A Good Way)
The heart of the internal combustion engine, you won’t be surprised to hear, is combustion.  A steam engine is an external combustion engine.  The fire burns outside the engine, to heat water and create steam which is stored under pressure and used to drive a piston.  By contrast, in an internal combustion engine, the combustion takes place inside the engine: burning air/fuel mixture expands and pushes on the piston which in turn pushes on the crankshaft, through the transmission to the driven wheels.  The mindblowing part of the internal combustion engine (which I will henceforth refer to as an engine) is the speed at which things take place.  Even at idle, which is approximately 900rpm, each piston is firing 7.5 times per second in a four cylinder engine.  The flame temperature inside the combustion chamber can exceed 2,000 deg C.  With so much happening, in such a harsh environment, it seems like a miracle that an engine can last long enough to reach the end of your driveway, let alone for hundreds of thousands of kilometres.
A quick recap of the four strokes of a four stroke engine:
Induction – The inlet valve(s) open and the piston moves down in the cylinder, creating a partial vacuum which draws in the air/fuel mixture
Compression – The inlet valve(s) close and the piston moves upwards in the cylinder, compressing the unburnt air/fuel mixture, and, thanks to the wonders of physics, heating it.
Ignition – The spark plug fires, igniting the air/fuel mixture which expands enormously and quickly, pushing the piston down in the cylinder, and turning the crankshaft.  (Diesel engines are different – they do not use a spark for ignition, instead diesel engines use the heating effect of compression to cause ignition.)
Exhaust – The exhaust valve opens and the piston moves upwards, pushing the burnt air/fuel mixture out.
These four cycles are informally and memorably referred to as suck, squeeze, bang and blow.
In this post, I will concentrate on the work of the spark plug.  Spark plugs are an ancient invention; they were invented in 1839, two years after Queen Victoria became queen, but it took until 1902 before a Bosch employee came up with a viable system for a spark ignition system.  An awful lot of spark plugs have been made since – Bosch alone have manufactured over 10 billion plugs.  Laid end to end, that’s 800,000km of spark plugs –  enough to stretch to the moon and back.  (Although spark plugs are very difficult to balance one on top of the other.)
When the air is cold and dry, you may notice static shocks occurring.  Typically you get out of your car, touch the door to shut it, and notice the zap.  That zap is several thousand volts equalising between you and the car.  You’re still alive because the current involved is tiny.  But that spark can (and in certain circumstances does) cause ignition.  And it is very similar to the spark in a spark ignition engine.
In the combustion chamber, compression raises the pressure to approximately ten times atmospheric pressure.  As pressure rises, it becomes harder for a spark to jump across the spark plug gap, so spark plugs require a voltage of around 40,000 volts.  It’s the job of the coil to increase the voltage from the battery’s 12 volts to the high voltage needed to create a spark in the combustion chamber.  The ignition timing system ensures that the spark fires at the right time to ensure optimum combustion of the air/fuel mixture, taking account of the speed of the moving piston and the time taken for the flame front to burn through the mixture.
Here is a video showing what happens inside a cylinder.  The camera is pointing upwards at the roof of the combustion chamber, with the inlet valve on the right and the exhaust valve on the left.  The shot jumps past the closing of the exhaust valve every time, so you don’t get to see the exhaust valve closing.
 For a wider view, here’s a video showing an engine built with a glass cylinder, which allows the internals to be seen clearly.  Interestingly there are a number of misfires.  The first is at 0:40, and there is another at 1:08.  (Remember suck, squeeze, bang and blow?)  An ignition (easy to see here occurs every second time the piston nears the top of its stroke. 
 The design of the spark plug has a central positive electrode running down the length of the plug.  The negative electrode is earthed via the cylinder head.  The ceramic insulator – the white bit – that runs the length of the spark plug separates the two electrodes.  Finally, the washer provides a gas-tight seal. 
As well as providing a spark (well….duh!), the spark plug helps maintain the combustion chamber at the correct temperature.  Different plug designs carry more or less heat away from the combustion chamber.  A plug that carries more heat away from the combustion chamber is referred to as a cooler plug, and one that carries less heat is referred to as a warmer plug.  A spark plug’s heat range is indicated by the number in the spark plug type.  For instance, the ‘5’ in a Bosch WR5DC+ or FGR5KQE0 spark plug indicates the heat range.  A plug containing a different number, e.g., WR7DC+ would have the same physical characteristics, but a different temperature rating.  And just to keep things exciting, different manufacturers use different numbers to convey the same information.  In other words, just because the appropriate Bosch plug for your car has a ‘5’ as the heat range indicator, it doesn’t mean that NGK will also use a ‘5’ for their equivalent. 
The other letters in a spark plug’s identifier tell you about electrode length, diameter, material and all sorts of other things, to a level of geeky-ness that no normal human being needs to know.  So let’s take a look at that, shall we, with the aid of this useful chart.  The info in the chart works well enough for the WR5DC+ plug, which is used in Porsche 911 cars to 1989.  And the info which comes out is:
W – Seat shape and thread:  20.8mm head with M14 x 1.25 thread
R – Plug design: Interference suppression resistor
5  - The heat range of the plug
D – Thread length and spark position
C – Type of electrode: Copper
+ – Marketing BS.  In this case ‘Super Plus Technology’
Like true love, but unlike herpes, spark plugs do not last forever.  Each time a spark plug creates a spark, a minuscule piece of material is removed from the electrode.  Eventually this material removal means that the plug no longer sparks correctly.  This is why spark plugs need to be replaced from time to time.  It is also why advice columnists and doctors are such busy people.


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All pictures and references to the brands and logos are for reference only, and do not imply any association with the brandholders. Most parts offered are OEM or quality aftermarket parts.  Genuine Porsche or MINI parts are specifically identified as such.  Master Parts  is not responsible for any typographical errors contained within the site.   Information within this website is for reference only.   It is your responsibility to verify that you are technically competent to carry out repair and maintenance procedures.  By entering this site, you agree to hold Master Parts free from any liability arising out of the use of any information contained within.
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