Khwarzimic Science Society - Automobile Mechantronics: Contants

Synopsis of a workshop on Automobile Mechatronics
For details of the workshop, click here

Direct Connection of the Radiator Fan

In PAKISTAN most of the cars have this problem that their radiator fan starts directly when the car is started regardless of the water temperature in the radiator, because the fan (electrically driven) circuit is by passed from the thermostat switch.

Well our mechanics do this because they think that by keeping the water temperature (indicated at the control panel by temperature gauge) to the minimum possible level will help engine perform well.

Actually the normal operating temperature of the engine, referred with respect to the water temperature in the radiator is 82 ± 3 °C. It is important to note that the water in the radiator boils at 115 to 120 °C .The radiator cap keeps the water under pressure and that is why the water boils above the its normal boiling point. The calibration of the gauge starts from 50°C .So the total range on the dial of the gauge is from 50°C to 120°C. So when needle reaches the middle of the gauge, engine is actually at its NORMAL WORKING TEMPERATURE {average temp of the gauge (as averages fall in the middle) is (115+50)/2 = 82.5 °C}. So at the normal working temperature the needle should be somewhere in the middle of the gauge. Whereas our so-called skilled mechanics interpret this as over-heating of the engine.THIS IS TOTALLY WRONG!

In a moment I will be listing the disadvantages of having your radiator fan direct .But first I would like to tell the readers about the cooling system of the engine in brief .

In water circuit of the engine (water circuit means the path of the water in engine) the effort is made to keep the water away from radiator till the water temperature in the water jacket reaches the 82°C mark. Simply you can say that water is not allowed to go in the radiator and is made to circulate inside the water jacket. This is done by using a device called THERMOSTAT VALVE. This thermostat valve when closed does not allow the water to circulate in the radiator. As soon as the temperature starts to go beyond 78°C the valve opens. This allows the water to be circulated in the radiator. If the heat dissipation of the radiator cannot keep the temperature at about 82°C the THERMOSTAT SWITCH completes the circuit of the electrically driven radiator fan to keep the temperature at 82°C by circulating air.

IT IS IMPORTANT TO NOTE that if temperature goes beyond 82°C it is comparatively acceptable as the most likely disadvantage is the increased potential of the engine to knock at higher gear and low engine rpm .

But if the temperature is less than the normal operating temperature then it will cause not only the deterioration of the engine performance but also shortening your engine life.

So in most of the cars the thermostat valve is removed, making the water circuit complete all the times and also bypassing the thermostat switch to keep fan running with engine regardless of engine temperature .

The problems that can arise as a result are discussed below:

Problems associated with Carburetion

Mixture Preparation

The petrol air/mixture preparation in the carburetor is a temperature dependent function. To keep the mixture preparation efficiency maximum the water in the radiator and water jacket is fed to the outer walls (around venturi) of the carburetor to keep it warm.

This increase of the temperature helps to mix up the petrol/air. If the temperature of the water is not at optimum, the petrol air mixture preparation in the carburetor will be effected .This will cause the engine to run on non-homogeneous mixture causing engine to misfire and vibrant engine operation is resulted.

One must have noticed the vibrant engine operation on cold starting in the morning. This is also due to the same reason as engine temperature is very low and mixture preparation is not good.

So by keeping the fan direct you are going make the engine run with vibrations and it will not deliver its full power as the mixture (petrol/air) is not homogeneous (well prepared) .

Mixture Distribution

If the temperature of the radiator water is kept low then the carburetor temperature also keeps low as the water circulates around the carburetor venturi (the cylindrical passage down the carburetor where air/petrol gets mixed) .The optimum carburetor operation is achieved at higher temperature. So by keeping the fan direct temperature is kept around 55 to 60 °C. This not only effects the mixture preparation but also the mixture movement as well. The mixture is prepared in carburetor and burnt in the cylinder. To get to cylinder the petrol/air passes through carburetor venturi and inlet manifold. If the temperature of venturi and manifold is low then the condensation of the petrol takes place. This will also cause the engine to run on non-homogeneous mixture and the engine running results will be the same as discussed in the above case .

It is important to note that if the mixture fed to the cylinder is not homogeneous then the combustion will not produced the desired amount of power .By meaning the homogeneous mixture it is assumed that the mixture strength expressed by weight ratios is same through out the mixture volume .The combustion is so fast that in non homogeneous mixture the petrol will not burn in the areas where not enough air is not present .This results in loss of fuel and engine produces less power .So by keeping your fan direct you will actually be doing the same thing as discussed above as water temperature will be low.

Problems associated with Engine

Four Stroke Cycle

As the engine temperature is not measured directly but is measured with respect to water temperature in the radiator so when the temperature of the radiator water reaches somewhere around 115 to 120°C, the temp. of the engine is enough to cause serious engine damage and it can even result in ceasing the engine .

The point here which I would like to emphasize is that engine temperature is never measured directly but the water temperature is measured. So if water temperature is low so is the engine temperature.The discussion to follow will be carried taking the radiator water temperature as reference.

When engine is started the temperature at the piston top or piston temperature is very low, same as atmospheric temperature. During engine operation it reaches hundred of degrees centigrade. No metal or alloy is developed yet which has zero expansion rate. So to accommodate the expansion the piston is made oval shaped. At cold starting the piston does not completely seal the cylinder, but as temperature rises the piston expands and seals the gap. At water temperatures above 115 to 120 °C the expansion is so great that the piston gets stuck resulting in serious engine damage. Below temperatures of say 75°C the piston is loosely fit in the cylinder.

We will discuss engine condition keeping in mind that water temperature as it is less than 75°C as fan is made to operate directly preventing the engine to operate at its normal working temperature.

Intake stroke

When the normal operating temperature of the engine is not achieved then the piston is not complete fit in the cylinder. During intake stroke piston descends from TDC(top dead center) and reaches BDC (bottom dead center). So if the piston is not completely fit than the petrol/air will not be sucked in properly. This will result in low engine volumetric efficiency

Compression Stroke

During compression stroke piston reaches from BDCto TDC. If the temperature of the piston is below normal then it will not fit the cylinder. So the petrol air sucked in prior ot compression, when compressed during compression stroke will bypass the piston and piston rings into the crankcase (It will be discussed in detail later on). So full compression is not achieved and this results in lower engine output and higher fuel consumption .

Power Stroke

During power stoke the rapidly expanding gases pushes the piston down giving power. If the piston does not fit completely then the expanding gases will by pass the piston rings thus resulting in incomplete power transfer making the engine H.P to decrease and fuel consumption to increase .

Exhaust Stroke

During exhaust stroke if piston is not complete fit then the exhaust will not be good. The exhaust air will by pass the piston, rings and will reach the crankcase causing oil fouling (it will also be discussed in detail later on) .

Engine Oil Fouling

(a) If the engine is made to run on low temp than optimum then the compression stroke is not fully efficient. The petrol/air mixture instead of being compressed, by passes the piston and piston rings. On its way to crankcase it passes the cylinder wall. The cylinder walls are soaked with engine oil to lubricate piton and rings. So when petrol by passes the piston rings it mixes with the oil on the cylinder walls. Petrol being excellent organic solvent dissolves and dilutes the engine oil, promoting engine wear .

(b) During power and exhaust stroke the burnt gases which are normally non metallic oxides (acidic in nature) bypass the piston rings and enter the crankcase. Here these gases mix with the water vapors. This forms acidic compounds that get mixed with engine oil resulting in altering the oil PH. It makes the engine oil acidic and corrosion always takes speeds up in the acidic medium.

For this reason the engine oils are made buffers (which can retain there PH although some amount of acid is added to them). But they can retain there PH only if a small amount of acid is added. So if engine is run consistently on low temperature then it will promote engine wear as engine oil will get acidic .

(c) The unburnt particles from the combustion chamber also by pass the piston as they are very small in size. When they enter the crankcase they get mixed with the oil. The engine oil are made detergents. That is they can accumulate small amount of these particles and hold them in suspension or you can say dissolve them. But this tendency is limited to a small amount. If prolonged use of engine at low temperature is carried out then it will cause pre mature engine wear.

As these particles do not completely dissolve in oil, they narrow down the small channels in engine lubrication circuit. This effects the engine lubrication as proper amount of oil is not fed to the moving parts resulting in engine wear.

Grading of Engine Oils

Engine oils available in market are classified according to their ability to maintain specific levels of fouling and dissolve the unburnt particles as they are detergent in nature.

Oils are classified as under

API SG/CD grade or
API SF/CD grade .

API stands for American Petroleum Institute.This grading (API SF/CD or API SG/CD ) standard is devised by API .

The SG or SF grade stands for the ability of the oil to handle the fouling.Out of two grades mentioned here the one with SG grading is better than SF. SG will hold more fouling than SF.Thus it has longer service life

It should be kept in mind that Engine Oil not only lubricates but it also has to sustain fouling caused by combustion.

Another important factor that you should keep in mind is the viscosity of the oil you are buying.The viscosity of the oil is determined by its SAE # ( SAE stands for Society of Automotive Engineers).The viscosity of the oil should be selected according to the climate in which you drive.The oil viscosity suited for our climate (where temperature ranges from 0°C to 50°C when starting the engine is also referred as AMBIENT TEMPERATURE) is SAE 20W-50 or SAE 20W-40 .

These are basically multigrade oils. There 20W part show that their viscosity falls under certain limit at -18°C (0°F)and the 50 part shows that the viscosity falls under certain limit at 99°C (210°F). Multigrade oil provide best protection at Low Temperature as well as at Elevated Temperatures.

Summarizing The Above Discussion

The following are the problems summarized keeping in mind that if engine is not running on its normal working temperature.

(1) It will decrease the air/petrol mixing efficiency .
(2) It will effect the distribution of petrol/air mixture to each cylinder.
(3) This will decrease the volumetric efficiency of the engine resulting in increased fuel consumption .
(4) The engine oil will get fouled prematurely resulting in more oil changes .
(5) The fouled engine oil will result in the premature engine wear shortening the engine life.
(6) Vibrant and rough idle (low speed running).
(7) Engine will never give its full power and torque .
(8) It will increase the fuel consumption without increasing the output of the engine .

Rear wheel Drive Cars

In rare wheel drive cars the radiator fan is not electrically driven but it is belt driven. It starts running as soon as the engine is started. To keep the engine temperature at optimum level the water is not allowed to circulate (when cold) in the radiator as the THERMOSTAT VALVE remains closed. As soon as the temperature starts to rise the valve is opened and the belt driven fan keeps the air circulating making the water temperature stick to an optimum level.

So in rear wheel drive cars the Thermostat valve should NEVER BE REMOVED. As it will result in the same problems as discussed above (by removing thermostat valve and making fan direct in front wheel cars) .

NOTE

It is important to note that one should never temper with the automotive systems as every part is installed for a specific purpose after very careful designing and testing. Every single part installed in the engine performs a specific operation. There are no spare or useless things installed. So disabling any one of them will greatly effect the engine performance and engine life

As discussed above, only by making the radiator fan direct you will encounter all of these problems . So instead of improving your engine performance you will decrease the engine out put and engine life will be shortened .