In recent correspondence the pros and cons of using starting fluid to assist in starting engines has been discussed. The intention of this article is to review the parameters, which enable a compression ignition engine to start unaided as per design and to examine the factors that may prevent this. As one correspondent pointed out the main reason, why starting fluids are required is usually as a direct result of poor levels of maintenance.

Tractors bought by enthusiasts or collectors, have in most cases reached the end of their economic life. They have probably spent the latter years of their life relegated to scraping the stockyard or other such mundane duties, a situation where poor starting is not a problem as there are other tractors and space available for a pull start. However, most enthusiasts require a tractor, which is capable of starting on the button.

Starting fluids contain volatile aromatics, petroleum products and upper cylinder lubricant, which assist combustion by lowering the ignition temperature. A Diesel engine does not have a sparking plug to ignite the fuel and create combustion, it relies on the action of the piston moving towards top dead centre, compressing the air in the cylinder and in turn heating it so that when the fuel is injected it ignites spontaneously.

This can be demonstrated when using a hand pump to inflate bicycle tyres, the harder one pumps the hotter the end of the pump becomes. This is due to the air absorbing the work done by the pump piston in compressing it. Conversely when compressed air (or other gases) expand it absorbs heat from its surrounding and creates the cooling effect experienced when using air driven tools which is the basis for refrigeration.

If we assume that, the air in the cylinder has been heated to the design temperature, in order for combustion to take place, the Diesel fuel must be injected into the cylinder as a fine mist or spray that enables it to mix with the air and ignite. A practical illustration of this is when lighting a paraffin blowlamp, where methylated spirit is poured around the burner assembly and lit in order to heat the paraffin and cause it to vaporise. If the pressure in the lamp is built up before, the paraffin has vaporised a jet of burning paraffin will result.

Fire fighting instructors use the “Triangle of Fire” to illustrate the conditions necessary for combustion; each side of the triangle is labelled as shown below and they demonstrate that by removing any one side of the triangle you will destroy the combustion process and extinguish the fire. The sides of the triangle also represent the conditions, which must be present in the cylinders of our engine in order for combustion to take place and for it to be able to start and run.

If we briefly examine the four-stroke compression ignition cycle, we find that these three conditions are met as follows. Air containing oxygen is drawn into the cylinder as the piston travels down with the inlet valve open; as the piston travels past bottom dead centre (BDC) the inlet valve starts to close. Once the valve is closed the air trapped in the cylinder is compressed as the piston travels towards top dead centre (TDC), depending on the designed compression ratio which may be between 16:1 or 20:1, the pressure in the cylinder rises to between 450 and 550 pounds per square inch.

This in turn heats the air to 350 to 450 degrees centigrade, which causes the atomised fuel, injected into the cylinder just before TDC, to mix with the air and spontaneously ignite. The self-ignition temperature of Diesel oils is in the region of 300-350 degrees centigrade. One important design consideration is the requirement for the air in the cylinder to swirl and in some engines, a combustion chamber is provided in order to achieve this, or the injectors may be furnished with several small orifices in order to disperse the fuel and so mix it with the air.

The burning air fuel mixture within the cylinder results in a sudden increase in pressure to some 700 pounds per square inch, that forces the piston, which due to the momentum of the flywheel etc. has just passed TDC, to travel downwards on the power stroke. The exhaust valve then starts to open just before BDC, the piston then continues upwards on the exhaust stroke; just before TDC the inlet valve opens and just past TDC the exhaust valve, closes and we have completed a full cycle on one cylinder.

When starting fluid is sprayed into the manifold of a compression ignition engine, its volatility and lower self-ignition temperature reduces the temperature at which the fuel air mixture ignites, enabling combustion to take place. This is repeated in each cylinder in turn causing the engine speed to increase, resulting in the air in the cylinders becoming hotter due to the increased rate of compression and thus enabling the injected fuel to ignite spontaneously allowing the engine to continue running unaided.

When starting a cold engine, a considerable amount of the heat generated by the action of the piston on the compression stroke is lost through the walls of the cylinder to the cooling water and the surrounding structure. The engine builders usually provide cold starting devices in order to compensate for this heat loss during the starting operation. This can be achieved by increasing the amount of fuel injected, heating the air in the manifold, fitting heaters or glow plugs to each cylinder or providing a means of preventing pressure building up in the cylinders (de-compression lever) whilst the engine is cranking until the engine is turning at a speed sufficient to sustain the conditions for combustion as discussed above.

Returning to our problem engine, the first requirement is air. Whilst a dirty or restricted air cleaner may well cause problems when the engine is running, possibly resulting in loss of power and a discoloured exhaust. Unless the air and exhaust ways are badly restricted it probably will not effect the starting of the engine!

The next requirement is to heat the air, which as we have seen is achieved by compressing the air in order to raise its temperature. During the start up this is dependent on two factors, the compression ratio and the rate at which the compression takes place or the cranking speed.

Any leakage past the piston or from the valves will result in a reduced compression ratio, which in turn will result in a lower maximum pressure accompanied by a corresponding lower temperature. This relationship is controlled by the laws of physics over which we have no control.

Bear in mind that leakage, which results in loss of compression, has a greater affect during start up with reduced cranking speeds than when the engine is idling. In the first instance, we should ensure that the inlet and exhaust valves are seating properly and that the clearances are correct. Severe leakage past the pistons can result in a pressurised crankcase, which can often be detected when the oil filler cap is removed.

The other important factor with regard to compressing and heating the air is the cranking speed, this is particularly so in cold weather with a cold engine. Under these conditions, there is a rapid transfer of heat from the air compressed in the cylinder to the cold metal surfaces of the cylinder walls and a high cranking speed is required, aided by the cold start devices fitted by the manufacturer, in order to compensate for these losses.

Most tractors are started by means of an electric motor, which after prolonged use can become lazy or sluggish. This may be due to broken or worn brushes, a worn or damaged commutator or a general breakdown of the insulation of the windings as a result of dirt and damp coupled with old age. Ideally, the overhaul of these motors should be left to a specialist, although the workshop manual will probably give details of how to strip them down and the work that can be carried out.

The starter motor is of course dependent on a battery to enable it to turn, so it is essential that the battery is in good condition and capable of maintaining the required cranking speed of the engine. In order to carryout checks on the battery and cables connecting it to the starter motor a multimeter or one of the cheap battery testers which are now available on the market is an essential tool. The battery testers, which use a series of coloured lights, are preferable as they indicate the state of charge/voltage of the battery under various conditions, by the illumination of the appropriate light

Do not be fooled by what appears to be a fully charged battery that gives a reading of 12.7 volts plus, as the voltage may fall well below the minimum battery voltage required of 10 volts when the engine is being cranked. Connecting the tester or meter between the motor terminal and earth when the engine is being cranked will indicate the voltage drop between the battery and the motor, which ideally should be zero. A large voltage drop could be as a result of dirty connections, dirty contacts in the starter solenoid, a damaged or undersized cable or damaged motor windings. Often the cause of the drop will become apparent due to the heat that occurs in the area of high resistance. The strap or cable connecting the battery to earth can also be a source of voltage drop and requires to be checked.

If the foregoing checks have not revealed any problems, we can assume that two of the sides of our triangle are in place and that air and heat are present in our cylinder. The last requirement is the supply of fuel, as the engine apparently runs normally once started, fuel starvation can be eliminated and we can assume that the fuel is reaching the injection pump and injectors and that the pump timing is correct.

It has already been stated that the fuel requires to be injected into the cylinder in the form of a mist or spray in order to ensure rapid and complete combustion. In order to do which the pump and injectors have to be properly maintained, the fine clearances of their various components dictate that only clean fuel should be used as the smallest particle of dirt can cause problems.

With the engine idling, pinch the injector pipes between finger and thumb; you should be able to feel a slight pulsing that corresponds with the injection of fuel. Some injectors enable a test pin to be fitted by removing the bonnet and inserting the pin so that it is contact with the nozzle valve, light finger pressure on the pin should detect a pulse if the injector is working correctly.

If you cannot detect a pulse, it may well mean that the injector is not operating correctly. Removing the injectors from the engine and reconnecting them to the pump so that they are pointing in such a way that the spray will not cause damage will enable you to monitor their performance. With the injectors removed, examine the tip to see if it is coated in carbon that indicates that the nozzle is dribbling; also check that the spray holes are clear.

With the throttle set to run, turn the engine over by hand or with the starter motor and watch the spray from the injectors. The fuel should leave the injector as a fine spray and with a burping noise; in multi hole nozzles, there should be an equal spray from each hole. If however it emerges as a stream or jet this indicates that the nozzle valve is probably sticking and requires to be overhauled. Do not be tempted to strip them down; unless you have a test pump and the necessary experience, the overhaul of injectors should be left to the specialist.

At this point, your attention must be drawn to the potential danger of atomised fuel. The fuel can leave the injector at a pressure of up to 3000 pounds per square inch and is capable of penetrating human flesh, which in turn can lead to blood poisoning and further complications. So, exercise great care when working on these systems to ensure that your hands and face are properly protected and that there are no naked lights in the vicinity.

Any problems with the pump may become apparent during the above test and it may be assumed that the injector is at fault; so in order to eliminate either one or the other the suspect injector should be replaced with one known to be operating correctly and the test repeated. Injection pumps are normally reliable if the fuel is clean and free of water, maintenance on these pumps is restricted to checking or changing the oil in the pump housing and examining the discharge valves. These valves, located in the pump discharge unions on the in line pumps, do occasionally stick or the springs may break. When removing them make sure that they are not mixed up as they are lapped to their body and are not interchangeable. Sticking valves or a broken spring will normally result in a noisy cylinder and smoking exhaust and may cause the injector to dribble.

If there should be any doubt as regards the performance of the fuel pump then it should be sent to a specialist for refurbishing. This is particularly true in the case of a working tractor or one used for ploughing competitions, conditions that require full power from the engine.

Having examined and rectified any faults found with the fuel system, we are in a position to complete the triangle with the addition of the third side, fuel.

Other factors that may affect the engine during start up are, using the wrong grade or too much oil in the sump resulting in additional drag, whilst disengaging the clutch will eliminate the drag of the transmission and so help the engine to turn easier.

Having established that the engine is cranking at the proper speed, the fuel is being atomised and the engine is breathing it should, with the assistance of the cold start device, start at the turn of the key. Should this not be the case, a decision has to be made as to whether to continue using a starting fluid or to strip the engine down and carryout further investigations.

In the short term, the use of starting fluid will overcome the starting problem, however its continued use will not alleviate the root cause of the problem but only help to mask it, until eventually a situation arises where the engine will either fail to start all together, or may start but fail to run.