cooling & lubrication system 

Whilst you are merrily blasting your way down the back straight and enjoying the thrill of it all at speeds approaching 100kph (or more in some cases), what’s furthest from your mind is that just behind your right elbow there are roughly 10,000 small controlled explosions taking place every minute.  Fuel is being burnt inside the combustion chamber, and hence the engine derived its other well-known name of ‘Internal Combustion’ or ‘IC engine’, from this process.

  

Efficiency of the engine

The thermal efficiency tell us how much of the chemical power contained in the fuel is converted into mechanical power at the crankshaft when measured on a dyno.  This efficiency is by no means earth shattering and is usually only around 20% for a two-stroke engine, but slightly more for a four-stroke.  In the case of a two-stroke engine, this low value of around 20% is largely dependent on the port timing and its scavenging ability.  In reality what this means is that somewhere around 80% of the available energy is not put to good use i.e. turning the crankshaft, but mostly ends up being turned into heat.  Roughly half of this heat is carried away by the exhaust gases, a similar amount goes into cooling the engine, and a small amount is used up by friction and ancillaries such as the cooling pump, or ending up as noise.  A typical energy split is as shown in the diagram below.

Not to be confused with thermal efficiency, we also have another measure of engine performance viz. mechanical efficiency.  In the days of the steam engine, a formula was developed to calculate the theoretical power the engine was capable of producing.  It was based on the pressure of the steam, the length of stroke on the crankshaft, the area of the piston, and the number of revs per minute – hence it was termed the PLAN formula being an amalgamation of the first letter of the names of each of the variables.  It is often referred to as the indicated power, whilst brake power is that measured when the engine is coupled to a dyno.  The mechanical efficiency is the ratio of these two values and varies between engines depending on their design, but is usually of the order of 80%, so it’s way higher than the thermal efficiency.

  

Cooling system requirements

The temperatures that the burning gases reach inside the combustion chamber and cylinder are upwards of 1 500°C, which is well above the melting point of the material that the cylinder head or the cylinder are made of.  Should this heat not be able to be dissipated, failure of the material is inevitable.

Additionally, we know that the fuel mixture being fed to the engine, apart from being burnt in the combustion chamber, ends up lubricating the crankshaft bearings, conrod bearing, gudgeon pin, piston rings, as well as leaving a film of oil on the cylinder walls.  Therefore if the heat of combustion is not adequately dissipated, this film of oil will quickly oxidize and deposit carbon onto the cylinder surfaces which will then result in a piston seizure.

Referring to the earlier diagram we can see that a good 40% of the total energy in the fuel goes straight down the exhaust pipe, so we only really need the cooling system to cope with the remaining 35% of the heat.  There are two types of cooling that we see on a kart engine viz. air cooling, or water cooling.

  

Air cooling

In this system, heat is conducted through the cylinder walls and cylinder head to their outer extremities where it is then transferred to the surrounding air via radiation.  To radiate this heat away efficiently, both the outsides of cylinder and the cylinder head have fins cast onto them.  The addition of these fins greatly increases the available area exposed to the air thus enhancing the transfer of heat to the surrounding atmosphere.  The rate of heat transfer is further enhanced when travelling at speed because of the velocity of the surrounding air travelling around or between the fins.  This is extremely necessary as the engine produces very little power at idle because it is only consuming a little fuel at that time.  This dramatically increases at racing speeds and wide throttle openings when the engine is consuming copious amounts of fuel, and getting a lot hotter as a result, and hence a rapid rate of heat transfer is required when this occurs.

This system of cooling has a number of advantages.  Firstly, it is a lot easier to design and manufacture.  It is also lighter than its water cooled counterpart because of the lack of water jackets, radiator and water pump.  It furthermore needs a lot less care and attention, and it doesn’t run the risk of frost damage (cracking of water jackets or radiator), water leaks from hoses, etc.  When designed correctly, an air cooled engine can also tolerate very high ambient temperatures such as those experienced in the desert – not that it concerns us much.  This type of cooling system is employed on the Comer C50, Kid-Rok and Mini-Rok engines.

  

Water cooling

There are a number of different types of water cooling systems, but the one used on karts is a forced circulation type that uses a centrifugal water pump.  A thermo-syphon system relies on the fact that as the hot water entering the top of the radiator cools down, its density changes and it sinks towards the bottom of the radiator.  As it does so, it is replaced by more hot water from the engine entering at the top.  The biggest problem is that the rate of water circulation within the system as a whole is extremely slow – far too low for our purposes.  The forced circulation system works on much the same principle, but there is a centrifugal pump added to it.  The pump is driven via a mechanical link to the engine (Rotax) or rear axle (Rok OKJ & DVS), and therefore speeds up the flow of liquid in the system pretty dramatically – as an example, the water pump on the Rotax 125cc engine provides a flow rate of about 22 litres per minute when the engine is running at 11,000rpm.

In this case, hot water from the cooling jackets around the cylinder and cylinder head is fed by the centrifugal pump to the top of the radiator, where it loses its heat via radiation to the atmosphere before being returned back to the engine to repeat the cycle.  The water pump is always located in the ‘cold’ leg of the system i.e. between the bottom of the radiator and the return feed to the engine.  The radiator is the active part of the heat exchange system and consists of an upper tank, a lower tank, and a set of tubes that connects the two.  The tubes which contain the water, are mounted vertically and pass through a large number of horizontal thin copper or aluminium sheets called fins.  Because of the large surface area created by the fins, the heat exchange to the atmosphere is greatly increased, and this is further enhanced by the forward motion of the kart when at speed.  This type of cooling system is used on all Rotax classes and also the OKJ and DVS engines used in the Rok series.  An adjustable flap (Rotax) or curtain (Rok) is also fitted in front of the radiator - its position can be altered to allow more or less air to pass through the radiator and is useful for quickly bringing the engine up to operating temperature.

 

Lubrication

Lubrication within the engine serves a variety of purposes.  No two surfaces are 100% smooth as there will always be irregularities caused by the method of manufacture, so when they are moving in close proximity to each other there is a certain amount of friction caused.  As the engine speed increases, so do the inertia forces of the rotating parts within the engine, and consequently the frictional losses do as well.  This increase is not linear, but is proportional to the square of the engine speed.

The magnitude of this friction is determined by a number of factors including the force applied to the mating surfaces, their surface roughness, type of lubrication between the surfaces, etc.  If you think that a ball bearing for example is 100% smooth, then think again because it’s only a question of what measuring instrument and magnification is used to determine this.  As a very crude analogy, a golf ball lying at the end of your garden when ‘measured’ with the Mk.1 eyeball from 20m away looks as smooth as you could hope for, but it’s another story altogether when right under your nose.  Lubrication is used to reduce the amount of friction and wear between two surfaces rubbing against one another.  In addition to the friction and wear, rubbing results in heat generation and also a loss in power.

 

Modes of lubrication

There are different modes of lubrication known as boundary layer, full-fluid, and mixed lubrication which is somewhat between the first two types and as shown in the diagram below.  Boundary layer is as the name implies – there is a thin layer of oil between the two surfaces, but they still make occasional contact.  In full-flow mode, the surfaces are kept completely apart by the layer of oil – this can be as a result of sufficient oil pressure provided via a pump, or caused as a result of a ‘wedge’ of oil being formed due to rotation between the two parts.  As an aside, you may have heard or read about air bearings – this is an example of full-flow ‘lubrication’ where high pressure air is used to keep the closely fitting parts apart, and is often used in extremely high speed applications e.g. a dentist’s drill.

Lubrication also aids in the removal of heat generated by the piston, cylinder and bearings by creating a cooling effect of the parts.  Furthermore, it acts as a sealing agent when the oil enters the gaps between the cylinder, piston and piston rings, thereby largely preventing leakage of the high pressure gases generated in the combustion process from entering the crankcase.

  

Types of oil

On a kart engine we don’t have the luxury of an oil pump or even an oil filter.  In fact, the lubrication system is effectively a total loss system.  Virtually all of the air/fuel mixture that contains the oil and enters the crankcase, is sent up to the combustion chamber via the transfer ports to be burnt and sent down the exhaust.  Only a small percentage of this atomized mixture, which contains anywhere between 20ml and 50ml of oil per litre of fuel, ‘remains’ within the crankcase to be used as a lubricant so it’s pretty important that a good quality of oil is used.

There are a number of lubricant types which can be obtained from animal fats, vegetables or minerals.  Oils derived from animal fats don’t stand up to heat well and end up being waxy or gummy, so they are not used on engines.  Oils from vegetables include castor oil, and they tend to be a bit gummy and also leave more sooty deposits behind when burnt compared to their mineral counterparts.  Mineral based lubricants are derived from crude oil occurring in nature and found in places such as the North Sea, USA, UAE, etc.

Additionally we have synthetic lubricants which are made of chemical compounds that are artificially made.  Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials.  For karting, the Bambino class as well as all the classes in the Rotax and Rok series are all regulated to use synthetic lubricants in very specific ratios.

  

Servicing of the cooling system

Air cooled engines require little or no maintenance to the fins as they seldom get clogged up.  On the other hand, a water cooling system does require some maintenance.  The most obvious are those that should be conducted on each and every race day and are essentially a visual check of the following:

  • Water level in the radiator, and condition of the cap and O-ring

  • Recheck the level when the engine is up to temperature and top up if required

  • Proper fitment of the radiator cap

  • Tightness of all hose clamps on the system

  • Leaks on the incoming and outgoing hoses to the radiator, the radiator core, the water pump connections, and the pump casing.

If you end up doing some ‘off-roading’ with the kart, then check the radiator fins are not bent – if so, they can be straightened using a long nose pliers.  Also check the radiator core for dirt ingress which can be removed using compressed air, but never a high pressure washer.  Finally, when the engine is being serviced, the engine builder should also check the condition of the drive gears for the pump, the drive shaft, and also the impeller.


Emile McGregor - MSA Technical Consultant