Engine Lubrication System

A portion of power is called friction power is lost to overcome the resistance to relative motion of all the moving parts of the engine. This includes the friction between the piston rings, piston skirt, and cylinder wall; friction in the big end, crankshaft, and camshaft bearings. Friction in the valve mechanism, friction in the gears, or pulleys and belts, which drive the camshaft and engine accessories.

The Importance of Lubrication

The lubricant and lubricating system perform the following functions:

1. Reduce the friction resistance of the engine to a minimum to ensure maximum mechanical efficiency.

2. Protect the engine against wear.

3. Contribute to cooling the piston and regions of the engine where friction work is dissipated.

4. Remove all impurities from lubricated regions.

5. Hold gas and oil leakage at an acceptable minimum level.

Lubricating System

In that system, oil is circulated through the system using an oil-pump, which may be driven directly from the crankshaft or indirectly from the camshaft or any auxiliary shaft. When the camshaft rotates, oil from the sump is drawn through the submerged strainer and pick-up pipe to the pump. The oil is then compressed and discharged through a drilling to the lubrication system. Control of the oil pressure is achieved by a pressure-relief valve situated on the output side of the pump. If the oil pressure becomes too high, the relief valve will open, bleeding any surplus oil back to the sump. From the oil-pump, all the oil flows through drillings in the crankcase to a cylindrical filter unit. The oil circulate around the filter bowl, forces its way through the center and flows out to the main oil gallery (the main oil passage). By various branch cross-drillings in the crankcase, oil is distributed to the crankshaft main-journal bearings and to the camshaft bearing.

Main-and big-end bearing lubrication:

Continuous oil feed to the big-end bearings from the oil grooves is provided by diagonal drillings in the crankshaft.

Cylinder and Piston Lubrication

One of the common methods for cylinder and piston lubrication is connecting-rod big-end radial-hole oil spray. In this method, through a small radial drilling in each connecting rod, a spray of oil is directed to the thrust side of the cylinder bore once every revolution.

Valve rocker-arm-mechanism lubrication

An oil drilling from one of the camshaft bearings supplies oil to the tappet-follower gallery drillings. Oil from this gallery flows through the hollow push-rod and to the rocker-arm.

Petrol-engine Carburetion Fuel System

Layout of a Petrol-engine Fuel System:

A fuel system fro a carburetted engine includes:

a) A fuel tank, which stores the petrol and has a fuel-gauge sensor unit incorporated to indicate the amount of petrol in the tank.

b) A feed pump, which transfers the petrol from the tank to the carburettor.

c) A feed filter, which prevents any contaminating particles from passing into the carburettor.

d) An air-silencer and filter unit, which quietens the fast-moving air intake and prevents dirt from entering the engine.

e) A carburettor, which merges air and petrol together so that they mixed in the correct proportions and the petrol is finely atomised.

f) An induction manifold, which collects the prepared air-fuel mixture and distributes it to the various inlet ports in the cylinder head.

g) Supply and return pipelines.

Carburetion

Air and petrol mixture strengths: According to chemical combination requirements, the air-fuel ratio, which gives complete combustion is 15. Rich mixtures, which contain more than the optimum amount of petrol, produce more power than optimum. The maximum power of the engine can be obtained when the mixture is about 15-20% rich (air-fuel ratio between 12 and 13). Prolonged running with very rich mixture will result in forming a black powder on the cylinder walls and on the spark-plugs. Weak mixture, which contain less than the optimum amount of petrol produce less power than optimum, but fuel economy is much better than for other conditions. For minimum fuel consumption, the mixture can be 15 to 20% weak (air fuel ratio is 17 to 18). Burning is generally slow and requires sufficient ignition timing advance to compensate for this prolonged combustion period.

Single-jet Fixed-choke Carburettor

It is a vertical tube that is connected to a petrol reservoir that has a float and a valve assembly as shown in the figure. Dividing the two sides of the U-tube carburettor at the base of the bent is a restriction orifice known as the petrol jet, its function being to meter the amount of petrol flowing into the venturi. As the air flow through the restriction its velocity increases and its pressure decreases so that the atmospheric pressure acts on the petrol in the float chamber, pushing the petrol into the venture. As the petrol enters the venturi it will immediately torn apart into various sizes of droplets so that the petrol will be finely distributed throughout the air stream as it is drawn into the engine cylinders.

To control the speed and load output of the engine, a butterfly throttle valve is placed on the downstream side of the venturi. The spindle of the valve is connected to the accelerator pedal by a cable or levers. The quantity of charge entering the engine can be varied by the degree of depression acting at the discharge nozzle, which depends on the angular position of the throttle spindle opening the butterfly valve. The function of the float chamber is to provide a reservoir of petrol of constant depth under steady-running conditions.

Limitation of the Single-Jet Carburettor

The quantity of air consumed by an engine in unit time is directly proportional to the engine speed, but due to the inertia of liquid flow, the rate at which petrol is drawn out of the discharge nozzle into the air stream increases almost with the square of the engine speed. Therefore, if the engine is designed to have the correct air-fuel ratio at the design speed (2000 rpm in the figure), the engine will have weak mixtures at speeds lower than the design speed so that the limitation of the single-jet carburettor is that it does not meet the engine requirement for the correct air-fuel ratio at speeds that are lower or higher than the design speed.

Capacity-well Compensation

During initial acceleration when the throttle is open, the pressure drop created at the venturi will draw fuel from the discharge nozzle at a far greater rate than can be supplied by the petrol jet alone, but the capacity well will provide the extra fuel demanded for rich acceleration mixture. The level of fuel in the capacity well will drop quickly until the well is emptied. Petrol droplets suspended in air will be formed at the base of the well and will prevent any more enriching of the mixture. Any further increase in speed will only result in a constant amount of fuel flow from the compensation petrol jet since the air passage bleed reduces some of the depression created across the petrol jet. The limitation of the capacity-well compensation is that it is not flexible enough under varying operating condition.

Air-bleed and capacity-well compensation

This system uses one fuel jet and suspension tube situated in the center of the capacity well. Under no-load conditions, the fuel level in the well will be the same as in the float chamber. With initial throttle opening, the fuel in the well will be consumed; thus, providing an enriched mixture. As the level of fuel in the well drops, it exposes the uppermost of the suspension tube holes. This allows more air to enter the well and mix with the fuel, thus preventing any tendency towards undue richness. As the petrol level in the well drops further it allow more air bleed correcting the composition of the mixture.

Coil Ignition System

The combustible mixture of air and petrol is ignited by a spark occurring between two electrodes in the combustion chamber at the end of the compression stroke just before TDC. It is the function of the ignition system to periodically provide a spark of sufficient heat intensity to ignite the mixture at the predetermined position in the engine’s cycle under all speed and load conditions.

The voltage necessary to ionise the air between the electrodes so that the spark will bridge the air gap can vary from as little as 500 volts when the gap is small and the engine is hot, to a value of 20,000 volts when the spark-plug electrodes are badly eroded, the air gap is large and the engine is cold.

Ignition-system equipment

Battery: This is usually a 6 or 12-volt battery. It stores chemical energy, which can be converted into electrical energy to supply the flow of current through the ignition system when required.

Ignition switch: This switch is connected in series in the coil primary-winding circuit. It enables the driver to switch on or off the electrical supply of the battery as required to operate the ignition system.

Ignition coil: It is an electrical step-up transformer, which converts the relatively low battery voltage to a high-intensity voltage.

If the ignition switch is closed and the contact-breaker points are together, current will flow from the battery through the primary winding and the earth-return path back to the battery so that a magnetic field is produced, which interlinks both the primary and secondary winding. When the rotating distributor cam opens the contact points, the primary current falls very rapidly to zero and the magnetic field also decays rapidly. Self-induction acts so as to oppose these changes and a very large back e.m.f. is induced in the primary winding. By transformer step-up action, an even larger e.m.f. (200 times larger) is thus induced in the secondary winding and is fed to the spark-plug gap to produce a spark.

Capacitor: The capacitor is connected in parallel with the contact-breaker points, the surge current in the primary winding when the contacts open finds an easier path through the capacitor so that the primary-current flow stops instantly and the back e.m.f. that is induced in the primary winding will be much greater. When the contacts close again, the charge stored in the capacitor charges into the primary winding and so helps to accelerate the build-up of a new magnetic field in the primary winding.

Spark-plug: It periodically provides a spark of sufficient heat intensity to ignite the charge mixture.