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Astraist
Master Driver



Joined: Mar 27, 2010
Posts: 209

PostPosted: Tue Nov 22, 2011 1:38 pm Reply with quote Back to top

This is the first technical article I write around here (and in general) about the car's mechanical parts. It will serve as a certain informative article to enrich knowledge, with a few points on the preferred driving style and the required maintainence. This part will be about the drivetrain: Engine and relevant systems.

The Motor
Car motors turn the heat and pressure of the burnt fuel into a movement that pushes down a piston. The piston than rotates a crank, which rotates a shaft. The shaft goes through the transmission, eventually rotating the wheels and tires, which moves the car, simple. The modern engine is a petrol or diesel running, four-stroke piston-based engine.

The four strokes stand for a "stroke" or stage during which the piston moves down and intakes fuel, than pushed down by the combustion of the fuel due to a spark, and than pushes up against the burnt fuel as it exits the combustion chamber through a second valve. In a diesel motor, the combustion compartment is filled with air that gets compressed, only than to be injected with pressuried diesel which ignites at once

The petrol engine burns it's fuel more quickly, so it rotates faster and reaches a higher RMP. A diesel engine is very powerfull, but limited to a lower RMP. We will not touch the Rotor engine (used only be Mazda) or a two-stroke engine at this oppurtunity.

The engine's actual "power" is measured as torque. The torque is the actual push that you feel and the force that rotates the wheels. Each engine experiences a peak amount of torque at a certain RPM (speed), and each engine builds up the torque differently, giving us a specific torque curve. Horsepower or, more accurately, brake horse-power (bHP) represents the car's final speed, rather than the force it can give.

The engine is built as a series of cylinders that host a piston that runs up and down. At the top end of the cylinder, well above the range of the piston, lay the valves that allow intake and exhaust of fuel, and the spark plug. The lower part, below the piston, opens up into a sump where the crank rotates and where the oil used as a lubricant drops down. The sump and combustion chamber are segregated by O-Rings between the piston and the wall of the cylinder.

Every car motor (to differ from many motorcycle engines) have a series of cylinders that work toghether. The cylinders can be lined up in various shapes, including a line that runs along the width of length of the car. Lining the cylinders differently allows to change the way in which the engine builds up heat, vibrates and allows to place it better in the car. For instance, the traditional motor that is placed along the length of the car is good at dissipiating heat and can be easily reached, but takes a lot of space, often into the very front of the car (having negative effects on the car's handling), and vibrates more.

In a V-shape engine, the vibrations of the opposite-placed cylinders cancles out most of the vibration, as it does in a W-shaped engine. These engines pack more heat at the intesection of the cylinders, but one can have a hard time placing them inside the car.

Valves and the Camshaft
The valves are operated by a camshaft that rotates and opens and closes them in rhythm. Modern engines often have a dual camshaft which operates two intake valves and one or two exhaust valves (as two little valves are better than one big valve). The problem is that at a higher RMP, the valves have to operate faster and faster to intake fuel and exhaust the burnt fuel, while still opening long and deep enough to intake and exhaust enough fuel to produce strong combustion and result in higher torque.

This is why modern cars don't have one fixed valve timing. They either have cam-phasing that allows the engine to operate at two, three or four different phases of speed, or have a continuous variable rate of opening the cams, which allows to keep the torque available over a wider range of RMP.

The camshaft also requires a lot of lubrication and cooling. This is a typical failure area of the engine's cooling system, where a crack at the head gasket causes coolant and/or oil to mix with each other and maybe even drop into the combustion chamber and burn along with the fuel (resulting in white smoke). It's also one of the critical parts in breaking-in a new motor.

Turbocharger
The turbocharger is made to boost the engine's natural characteristics with much more torque, but makes it recieve this extra torque rather ubruptly, just as the turbo itself kicks in. The turbo simply puts more air into the engine and thus allows it to burn fuel better. A turbo-charger is a turbine that rotates off of the car's exhaust fumes and pushes another, geared turbine which compresses air into the engine.

The problem is that the compressor has a tendency to kick in very suddenly. At a lower engine RPM the turbo simply isn't effective. Even at the effective range of RPM the engine loses compression all at once if you let up the gas. In turbo-charged rallly cars in the ninties, left foot braking became very popular due to the use of very complex turbochargers. Modern turbochargers have variable blade angles at the turbine which allows it to operate effectivelly at a wider range of RMP.

A supercharger works at the same way, but is actuated by a belt tied to the engine. Therefore it can begin to operate more quickly (i.e. at a lower engine speed) but will take away some of the engine's force, and be limited to lower RPM's.

An addition to both superchargers and turbochargers is an intercooler which helps cool the air intake half-way between the charger and the engine. Hot air loses it's compression and sadly, air heats up dramatically inside a charger, so an intercooler is very effective, especially when outside ambient temperatures are high.


Lubrication System
Lubrication of the engine is necessary at the contact between the piston and cylinder wall and O-Ring, as well as around the camshaft and inside hydraulic valves. For this, we fill and check motor oil. The motor-oil has two representive figures, typically with a W between them, e.g. 20W40. Modern motor-oils are multi-graded. They have additives that allow them to change their viscosity according the temperatures they operate in (which also depend somewhat at external temperatures).

The first figure shows how well the oil operates at a low temperature. This is relevant when you just start the car. The oil needs to be thick enough to protect the engine, especially where oil remains on the parts of the engine, but thin enough to run through the tubes and the narrow parts and run through the engine and lubricate it. At high working temperatures inside the engine, the second figure suggests how thick the oil can remain at working temperatures, which again should be enough for it to protect the engine from harm, but not too thick, so it won't make the engine work too hard needlessly. "W" stands for the minimal requirements for winter use in the USA (a certain viscosity at 0 degrees farenheit).

The oil also has an API rating that is marked either S for petrol or C for diesel, with a second letter going up the alpha-bet marking the grade of the oil, where modern engines use grades like SH or SJ. Other pieces of information include the Viscosity Index, which shows how gradually the viscosity changes. Two other pieces of data grant you the oil's Flash Point and Fire Point, the first states when the oil might produce a flame, and the second, about 50 degrees higher, would state a point where the oil itself starts to burn away. The last state, is the Sulfated Ash, which shows just how much ash be left after the oil has burnt away.

The oil can be mineral, synthetic or semi-synthetic. For the well-being of your engine, go for synthetic, unless the motor cannot withstand it. A synthetic oil has all of the additives put into the molecules of the oil, rather than be composed from a base oil (which comes from Petroleum), combined with additives and un-filtered residues. The problem with the latter, is that the additives themselves tend to break away and burn down into sludge.

Oil should be checked, preferably weekly, when cold. It should be replaced at least as early as required by the car's manufacturer. In practice, the tests that conclude how quickly the oils needs to be changed are based almost solely on driving down a highway at reasonable ambient temperatures. Higher external temperatures, problems with other car parts, dust and air pollution, as well as a driving style of "stop and go" which is not uncommon when driving in towns, breaks down oils much faster.

Many modern cars don't even get to move around to a point where the engine or oil heats up to working temperatures. In these conditions, go for the "heavy duty" recommendations. If you don't, the engine could pack up sludge amazingly quickly within a few years. You can try to flush the engine, but much of the sludge will simply drop into the oil pan and get sucked up the oil filter. There will also be soot in the combustion chamber and further sludge that might stick the O-Rings to the pistons. In this case you would even need to go as far as opening the oil pan and scraping it from sludge.

Old engines (say, after eight years or 38,000 miles), it's a good idea to flush the engine anyway and maybe even open up the oil pan in search for sludge anyway. Be warned, if the engine hasn't been well-maintained to begin with than this might suddenly "reveal" problems that the sludge "covered" like leaking piston rings.

Putting too much oil into the engine is perhaps even worst. Too little oil is not going to cause any immediate damage. Too much oil will form such pressure that could cause the U-Rings to burst, or build up a small pool of oil down the sump, causing it to ressist the rotation of the crank, which by itself would tear down the oil, making foam in it.

Pollution
A petrol engine isn't supposed to pollute the air at all! The exhaust should only exhaust CO2 and water droplets. However, no modern combustion engine has reached 100% efficiency. The gas that is being taken out of the engine is in fact fuel that has either been semi-burnt (Carbon Monoxide) or not burnt at all. This is caused by some of the oil being quenched against the sidewall of the cylinder, or by being absorbed into the spongy soot that builds up inside the combustion chamber.

Breaking In the Motor
It's no myth! As well as modern engines are made and tested, they still require some breaking-in. It is nothing like the breaking-in procedure in old cars, where mineral oils were used and the car was driven very differently during a period of several thousands of miles. Howeve, there is still some bedding-in to conduct: The O-rings should be worn down the just the right size, as should the camshaft. Even a small buildup of soot is important because it actually helps the rings become sealed. The ECU also needs time to learn your driving style.

The most crucial phase is the first sixty miles or so, where it's important to let the RPM stabilize after starting the car (about 20 seconds before touching the gas), and than start driving gently and at a low RPM untill the car reaches working temperature. It's important not to cruise down the highway at a constant speed, because this might glaze the rings that need to be breaked into. Also avoid heavy loads. Once the engine reaches working temperatures, increase the load unto it. Afterwards, maintain the same rules for the next 1,500 miles or so, but try to bring the car up to higher and higher RPM once it reached working temperatures. Make your first oil change a bit early, because the oil will pack up a lot of metallic residue from this period.

These rules are also good for when the motor in general: Giving the RMP about 15 seconds to stabilize before you touch the gas, drive it gently untill it heats all the way up, and than use the time to increase the load on the engine for a few minutes each day. Once in a while, a cruise down the highway at a constant, low RPM can do wonders to an engine by clearing out soot.
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Astraist
Master Driver



Joined: Mar 27, 2010
Posts: 209

PostPosted: Sat Dec 03, 2011 12:05 pm Reply with quote Back to top

The intake manifold
The proper intake of air is one of the most important parameters in the function of a good engine. For ages manufacturers worked with short manifolds, only to discover that it was in fact a longer manifold that would generate a "pendulum" effect that allowed the air pressure to build up better.

The problem is that this effect only works compatible with a certain RPM. The idea is that the valve is closed and the air is pushed back, and than back inside. If the valve opens at just the peak of the pendulum, more torque can be achieved. This is why variable intake manifolds, with different routes for different RPMs are used in modern vehicles.

The exhaust
Works much like the intake. The idea is for exhaust gases to run out of the pistons in rhythm so they create a suction force that pulls the next round of fumes through and for this, the different lines should be twisted and equally long.

The width of the manifold also changes the equation by forming a narrow area (mostly at the very end of the manifold) which creates back-pressure which limits the torque curve. This effect is reduced by use of variable exhaust manifold which can run the fumes through either one of two or three tubes of a varying radius. A backpressure also has an advantage of stopping the RPM from dropping all that suddenly upon letting up the throttle.

Modern exhausts have a catalytic converter which filters out poisonous fumes as well as unburnt oil residues and other problems. It is often replaced by a free-flowing exhaust to increase the engine's torque.

The cooling system
The engine is divided into a big, rigid block which holds the pistons and crankshafts, an engine head which holds the camshaft and valves and a thin gasket in between. The block and mainly the head holds passages for lubricants and mainly for coolant. Any overheating will cause cracking in the gasket or head and lead to oil breaching into the coolant, oil and/or coolant breaching into the camshaft or cylinders, etc...


The Clutch
The engine's speed of rotation can only produce so much speed. In order to reach higher speeds while still having available torque, you need a transmission. The transmission is connected to the engine by a flywheel which is connected to a clutch.

The clutch is simply built out of two plates which can be pressed against each other. This allows them to convey torque from the flywheel to the transmission. The plates have a high-friction material which generates grip much like tires on tarmac. The more grippy the material, the more immediate is the connection. This has a disadvantage in situations where you need to let up the clutch only partially when parking or manuevering with the clutch semi-engaged.

Automatic transmissions have a torque converter in which the two plates are replaced with rotors that swirle around a hyradulic fluid. This allows for the "clutch" to be in a constant state of slippage, allowing for the transmission to shift gears without requiring the driver to let up the throttle as a clutch is engaged. The disadvantage is in weight, wear of the transmission lubricant and massive loss of torque, which is reduced at high speeds by engaging a rigid link through the torque converter. An automatic transmission also has a series of clutches inside it, to engage or disangage specific gears, as well as bands in some transmissions.

The transmission
The transmission itself is made out of a series of gears that interlock with a gear that connects to the driveline. These gears offer changing ratios that allow to drive at higher speeds for a given amount of torque. Modern gears have syncros which are effectivelly sleeves that allow the driver to shift gears more smoothly, especially down to or up from first gear.

Automatic transmissions do the same by use of complex planetary gearsets that involve a "sun" gear in the middle, three planet gears and a ring gear, that allow to form various gear ratios. Modern transmissions will normally have two or three such gearsets to create a wide range of gears. The downside is the much more complex mechanism, and the mechanical problems in producing the proper gear ratio for second gear, where the planetary gear set actually creates a negative gear ratio which has to be reversed. Another difference is that on a manual transmission, reverse gear allows for the most torque, where in an automatic it offers less torque than first gear.

Shorter gearing ratios means that the transmission will support a smaller range of engine speeds, but allow the torque to build up more quickly. This has the advantage of more immediate power, but more wandering through the gears and more gas consumption. Longer ratios offer a wider range of effective RPM per gear, which allow to drive at different rev speeds, like a slower engine speed for better milleage, with the downside of less immediate torque.

The Differential
The last stage in the driveline is the differential, which adds a final gear ratio and a distribution of force between the left and right wheels. The idea is that the inner wheels has to take a tigher radius and rotate more slowly. Therefore, if the force is distributed 50\50, it would create tension that will wear on the tires and the axle, especially at tight turning (try to manuever in a tight parking space with a locked differential and hear the axle screaming).

A differential is a complex gearing that allows some flexibility between the wheels. The problem is when turning, weight is transferred from one side to the other. The result might be that the inside tire will have little grip and in that case, it might lose traction under acceleration and spin. In this situation the differential will move 100% to the spinning wheel, which I believe leaves zero power for the wheel that actually grips.

This is the reason for inventing differentials that hold clutches which lock up the two wheels when there is too much slip on one side. These include torque-sensing differentials or proper limited-slip differentials, which allow to use up all of the traction of both tires under acceleration and especially result in highly controled power slides in rear-wheel driven cars.

Without such a differential, left foot braking can be used to apply the brakes against the driving wheels and prevent them from spinning. The same technique can be used to "fool" a limited-slip differential because it will "interprate" the ressistance of the brakes as slip and lock up.

Front wheel driven cars have spools which have an outer dumbell joint which allows them to pivot as the front wheels are titled into corners, therefore putting a certain limit to the amount of horsepower that can be applied unto the wheels in such a car and makes room for possible complication, especially as the grommets are worn off and dirt comes in. A rear-wheel drive isn't all that better, since it has a long driveshaft which takes up space and changes the weight distribution and leads to further wear and loss of power. An all-wheel drive has even more parts and thus more weight and wear.
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