STARTING SYSTEM: COMPONENTS AND WORKING PRINCIPLES

The engine can’t “start” rotational movement on its own. It needs an electric motor to get it up to a minimal RPM to run, then the engine can run under its own power. The starter is the biggest load on the vehicles electrical system. We cannot simply run all that current through the ignition switch, in most systems a relay is used to activate the starter solenoid, and the starter solenoid itself acts as another relay to engage the starter motor (explained later). Before electric starters, automobile owners needed to crank the engine over themselves! This was not ideal for any kind of quick getaway.

The starter motor is an electric motor that rotates your engine in order to allow the spark and fuel injection systems to begin the engine's operation under its own power. Typically, the starter is a large electric motor and stator coil mounted to the bottom (generally to one side) of the vehicle's transmission bell housing where it connects to the engine itself. The starter has gears which mesh with a large flywheel gear on the backside of the engine, which turns the central crankshaft. Because this is a lot of physical weight and friction to overcome, starter motors are generally powerful, high-speed motors and use an ignition coil to ramp up their power before engaging.

COMPONENTS OF STARTING SYSTEM

1. Battery

The automotive battery, also known as a lead-acid storage battery, is an electrochemical device that produces voltage and delivers current. In an automotive battery, we can reverse the electrochemical action, thereby recharging the battery, which will then give us many years of service. The purpose of the battery is to supply current to the starter motor, provide current to the ignition system while cranking, to supply additional current when the demand is higher than the alternator can supply and to act as an electrical reservoir.

2. Ignition Switch

The ignition switch allows the driver to distribute electrical current to where it is needed. There are generally 5 key switch positions that are used:

1. Lock- All circuits are open ( no current supplied) and the steering wheel is in the lock position. In some cars, the transmission lever cannot be moved in this position. If the steering wheel is applying pressure to the locking mechanism, the key might be hard to turn. If you do experience this type of condition, try moving the steering wheel to remove the pressure as you turn the key.

2. Off- All circuits are open, but the steering wheel can be turned and the key cannot be extracted.

3. Run- All circuits, except the starter circuit, are closed (current is allowed to pass through). Current is supplied to all but the starter circuit.

4. Start- Power is supplied to the ignition circuit and the starter motor only. That is why the radio stops playing in the start position. This position of the ignition switch is spring-loaded so that the starter is not engaged while the engine is running. This position is used momentarily, just to activate the starter.

5. Accessory- Power is supplied to all but the ignition and starter circuit. This allows you to play the radio, work the power windows, etc. while the engine is not running.

Most ignition switches are mounted on the steering column. Some switches are actually two separate parts;

* The lock into which you insert the key. This component also contains the mechanism to lock the steering wheel and shifter.

* The switch which contains the actual electrical circuits. It is usually mounted on top of the steering column just behind the dash and is connected to the lock by a linkage or rod.

3. Neutral Safety Switch

This switch opens (denies current to) the starter circuit when the transmission is in any gear but Neutral or Park on automatic transmissions. This switch is normally connected to the transmission linkage or directly on the transmission. Most cars utilize this same switch to apply current to the backup lights when the transmission is put in reverse. Standard transmission cars will connect this switch to the clutch pedal so that the starter will not engage unless the clutch pedal is depressed. If you find that you have to move the shifter away from park or neutral to get the car to start, it usually means that this switch needs adjustment. If your car has an automatic parking brake release, the neutral safety switch will control that function also.

4. Starter Relay

A relay is a device that allows a small amount of electrical current to control a large amount of current. An automobile starter uses a large amount of current (250+ amps) to start an engine. If we were to allow that much current to go through the ignition switch, we would not only need a very large switch, but all the wires would have to be the size of battery cables (not very practical). A starter relay is installed in series between the battery and the starter. Some cars use a starter solenoid to accomplish the same purpose of allowing a small amount of current from the ignition switch to control a high current flow from the battery to the starter. The starter solenoid in some cases also mechanically engages the starter gear with the engine.

5. Battery Cables

Battery cables are large diameter, the multi-stranded wire which carries the high current (250+ amps) necessary to operate the starter motor. Some have a smaller wire soldered to the terminal which is used to either operate a smaller device or to provide an additional ground. When the smaller cable burns, this indicates a high resistance in the heavy cable. Care must be taken to keep the battery cable ends (terminals) clean and tight. Battery cables can be replaced with ones that are slightly larger but never smaller.

6. Starter Motor

The starter motor is a powerful electric motor, with a small gear (pinion) attached to the end. When activated, the gear has meshed with a larger gear (ring), which is attached to the engine. The starter motor then spins the engine over so that the piston can draw in a fuel/ air mixture, which is then ignited to start the engine. When the engine starts to spin faster than the starter, a device called an overrunning clutch (Bendix drive) automatically disengages the starter gear from the engine gear.

STARTER MOTOR PARTS

1. Starter Solenoid

The starter solenoid sits on top of the starter motor and performs two main functions, it acts as a heavy-duty relay for the starter and it engages the starter pinion gear to the ring gear on the flywheel/flex-plate/torque converter. The solenoid has 3 terminals; a B+ terminal, an S terminal, and an M terminal. The B+ terminal is connected directly to the battery positive at all times. This wire is infused meaning that if there is a short to ground on this wire, there will be sparks until the battery is drained. The wire from the battery to the B+ terminal will be very thick because it needs to carry call the current necessary to turn the starter motor and overcome engine compression. The S terminal receives power from the ignition switch either directly or indirectly with a relay. The S terminal connects to two winding, the pull-in winding and the hold in the winding. These winding are simply coils of wire wrapped around a plunger, which when energized produce and electromagnet. The pull-in winding is made up of thicker winding and creates a strong electromagnet. It is grounded through the M terminal and starter motor. The hold-in winding is smaller and creates a weaker electromagnet. It is grounded directly to the starter case. The plunger sits in the middle of the winding and is held in place by a spring. The plunger gets pulled/held in by the winding when they are energized. At one end it is connected to a lever which forces the starter pinion gear to mesh with the ring gear. At the other end, when the plunger reaches the end of its travel, it pushes a contact disk which connects the B+ terminal to the M terminal which is connected to the starter motor. This energizes the starter motor and also causes the pull-in winding to stop flowing power. This is because once the contact disk connects B+ to M there is 12v on both sides of the pull-in winding and no ground. The hold-in winding continues to flow electricity and holds the plunger in place until the key is returns to the run position. The solenoid needs both windings to pull the plunger in but only the hold-in winding to keep it there. It takes much more effort to move the plunger to engage the starter than it does to hold it there. Since the pull-in winding is no longer necessary, it would only waste electrical power to continue to power it.

2. Starter Motor

The starter motor converts electric energy into rotational motion, using electromagnetism or electromagnetic repulsion. Most starters used in automotive today are permanent magnet starters. These starters have several permanent magnets placed inside the case around an armature. An armature is used to make an electromagnetic field of the same polarity as the permanent magnets, causing the armature to repel the magnets. Power from the M terminal and ground from the case is supplied to the commutator strip through the brushes. The commutator strips Are connected to each other through the armature windings, this causes an electromagnetic field to form around the armature strips that are flowing power. If power is fed to commutator strip 1, the ground is on commutator strip 5, power will have to travel through armature strips 2,3, and 4 to get to commutator strip 5. This will create a magnetic field around armature strips 2,3 and 4. To get the armature to rotate, a permanent magnet is placed near, but not right on top of where the electromagnetic field is formed. When the two like polarities repel, the armature begins to rotate. As the armature rotates, the brushes will contact the next commutator strips, keeping the electromagnetic field in one place (just next to the permanent magnet) but allowing the armature to spin. This is what creates the rotational movement necessary to start the engine. Starters may also have a planetary gear-set to reduce RPM and increase torque to the ring gear. Heavy-duty starters use field coils instead of permanent magnets. Basically, they make both magnetic fields using electromagnetism instead of relying on permanent magnets. These starters are much more powerful than a permanent magnet starter but they take up more space, are much heavier and cost more to produce.

3. Starter Drive Pinion

The starter drive pinion is held out mesh with the ring gear by a spring until the starter solenoid engages and moves the lever, pushing the starter pinion into mesh with the ring gear. When the engine starts, the operator allows the key to return to the run position. This cuts power to the starter solenoid, which allows the spring to push the plunger back to its normal position. The plungers lever will pull the starter drive pinion back, out of mesh with the ring gear. It is important that the starter drives the flywheel and not the other way around. This is why starter drives have a one-way clutch. The one-way clutch allows the starter to turn the flywheel, but if the flywheel starts to cause the starter pinion to turn faster than the armature, the one-way clutch will slip. This protects the starter from spinning too fast.

WORKING PRINCIPLES

🗝To make an engine start it must be turned at some speed, so that it sucks fuel and air into the cylinders, and compresses it.

🗝The powerful electric starter motor does the turning. Its shaft carries a small pinion (gear wheel) which engages with a large gear ring around the rim of the engine flywheel.

🗝In a front-engine layout, the starter is mounted low down near the back of the engine.

🗝The starter needs a heavy electric current, which it draws through thick wires from the battery. No ordinary hand-operated switch could switch it on: it needs a large switch to handle the high current.

🗝The switch has to be turned on and off very quickly to avoid dangerous, damaging sparking. So a solenoid is used - an arrangement where a small switch turns on an electromagnet to complete the circuit.

🗝The starter switch is usually worked by the ignition key. Turn the key beyond the 'ignition on' position to feed current to the solenoid.

🗝The ignition switch has a return spring so that as soon as you release the key it springs back and turns the starter switch off.

🗝When the switch feeds current to the solenoid, the electromagnet attracts an iron rod.

🗝The movement of the rod closes two heavy contacts, completing the circuit from the battery to the starter.

🗝The rod also has a return spring -when the ignition switch stops feeding current to the solenoid, the contacts open and the starter motor stops.

🗝The return springs are needed because the starter motor must not turn more than it has to in order to start the engine. The reason is partly that the starter uses a lot of electricity, which quickly runs down the battery.

🗝Also, if the engine starts and the starter motor stays engaged, the engine will spin the starter so fast that it may be badly damaged.

🗝The starter motor itself has a device, called a Bendix gear, which engages its pinion with the gear ring on the flywheel only while the starter is turning the engine. It disengages as soon as the engine picks up speed, and there are two ways by which it does so - the inertia system and the pre-engaged system.

🗝The inertia starter relies on the inertia of the pinion - that is, its reluctance to begin to turn.

🗝The pinion is not fixed rigidly to the motor shaft - it is threaded on to it, like a freely turning nut on a very coarse-thread bolt.

🗝Imagine that you suddenly spin the bolt: the inertia of the nut keeps it from turning at once, so it shifts along the thread of the bolt.

🗝When an inertia starter spins, the pinion moves along the thread of the motor shaft and engages with the flywheel gear ring.

🗝It then reaches a stop at the end of the thread, begins to turn with the shaft and so turns the engine.

🗝Once the engine starts, it spins the pinion faster than its own starter-motor shaft. The spinning action screws the pinion back down its thread and out of engagement.

🗝The pinion returns so violently that there has to be a strong spring on the shaft to cushion its impact.

🗝The violent engagement and disengagement of an inertia starter can cause heavy wear on the gear teeth. To overcome that problem the pre-engaged starter was introduced, which has a solenoid mounted on the motor.

🗝There's more to a car starter system: As well as switching on the motor, the solenoid also slides the pinion along the shaft to engage it.

🗝The shaft has straight splines rather than a Bendix thread so that the pinion always turns with it.

🗝The pinion is brought into contact with the toothed ring on the flywheel by a sliding fork. The fork is moved by a solenoid, which has two sets of contacts that close one after the other.

🗝The first contact supplies a low current to the motor so that it turns slowly - just far enough to let the pinion teeth engage. Then the second contacts close, feeding the motor a high current to turn the engine.

🗝The starter motor is saved from over-speeding when the engine starts by means of a freewheel clutch, like the freewheel of a bicycle. The return spring of the solenoid withdraws the pinion from engagement.

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SHOCK ABSORBERS/ DAMPERS: WORKING PRINCIPLE, CLASSIFICATION, AND FUNCTIONS

Shock absorbers are basically oil pumps. A piston is attached to the end of the piston rod and works against hydraulic fluid in the pressure tube. As the suspension travels up and down, the hydraulic fluid is forced through tiny holes, called orifices, inside the piston. However, these orifices let only a small amount of fluid through the piston. This slows down the piston, which in turn slows down spring and suspension movement.

All modern shock absorbers are velocity-sensitive hydraulic damping devices – meaning the faster the suspension moves, the more resistance the shock absorber provides.
Because of this feature, shock absorbers adjust to road conditions. As a result, shock absorbers reduce the rate of:

• Bounce
• Roll or sway
• Brake dive and Acceleration squat

Shock absorbers work on the principle of fluid displacement on both the compression and extension cycle. A typical car or light truck will have more resistance during its extension cycle then its compression cycle. The compression cycle controls the motion of a vehicle's unsprung weight, while extension controls the heavier sprung weight.

FUNCTIONS OF DAMPER

The main function of the shock absorber is to absorb the shocks and damp them as soon as possible so that a smooth ride can be obtained.

Some other important functions of the shock absorber are
 It limits vehicle body movement
 It stabilizes our ride as discussed above
 It stabilizes vehicle tires which are disturbed due to sudden shock, hence it is very important for safety purpose also
 It also minimizes tire and bodywear of the automobile and hence reduces overall maintenance cost
It may sound a simple job but this is the main thing on which the comfort level of your ride depends.

WORKING PRINCIPLE

To understand the shock absorber, it is very important to understand its working.

First of all, we should know that there are generally two types of shock absorbers one is hydraulic and another one is pneumatic. However, working of both the types of shock absorbers is same.

A shock absorber is generally coupled with a spring, which converts sudden shock waves into oscillatory motion. This oscillatory motion gives us instant relief from the shock but, nobody can have his or her whole ride with these oscillations.

Here is the need for shock absorber arises, it is used to damp those oscillations which are made by the springs.
A general shock absorber contains a perforated piston in a hydraulic chamber. The chamber is totally sealed and hence if piston has to make some movement the only way is to let the hydraulic liquid pass through it.

When a shock comes, the piston has to move due to shock. When the piston moves than the hydraulic liquid in the shock absorber has to pass through it.

When the liquid is passed through the very tiny perforated holes in the piston the piston has to do some work against it. That work is done on that expense of the energy generated due to the shock and hence soon the shock absorber loses all the shock energy, which results in no oscillation and smooth ride.

SHOCK ABSORBER DESIGN TYPES

There are several shock absorber designs in use today:
1. Twin Tube Designs

• Gas Charged
• PSD (position sensitive damping)
• ASD (Acceleration Sensitive Damping)

2. Mono-Tube

A. Twin-Tube – Gas Charged Design

The prime function of gas charging is to minimize aeration of the hydraulic fluid. The pressure of the nitrogen gas compresses air bubbles in the hydraulic fluid. This prevents the oil and air from mixing and creating foam. Foam affects performance because it can be compressed – fluid can not. With aeration reduced, the shock is able to react faster and more predictably, allowing for quicker response time and helping keep the tire firmly planted on the road surface.

Advantages:
• Improves handling by reducing roll, sway, and dive
• Reduces aeration offering a greater range of control over a wider variety of road conditions as compared to non-gas units
• Reduced fade – shocks can lose damping capability as they heat up during use. Gas-charged shocks could cut this loss of performance, called fade

B. Twin Tube – PSD Design

Ride engineers had to compromise between soft valving and firm valving. With soft valving, the fluid flows more easily. The result is a smoother ride, but with poor handling and a lot of roll/sway. When valving is firm, fluid flows less easily. Handling is improved, but the ride can become harsh.
With the advent of gas charging, ride engineers were able to open up the orifice controls of these valves and improve the balance between comfort and control capabilities available in traditional velocity sensitive dampers.
A leap beyond fluid velocity control is an advanced technology that takes into account the position of the valve within the pressure tube. This is called Position Sensitive Damping (PSD).
The key to this innovation is precision tapered grooves in the pressure tube. Every application is individually tuned, tailoring the length, depth, and taper of these grooves to ensure optimal ride comfort and added control. This in essence creates two zones within the pressure tube.
The first zone, the comfort zone, is where normal driving takes place.
The second zone, the control zone, is utilized during demanding driving situations.

Advantages:

• Allows ride engineers to move beyond simple velocity sensitive valving and use the position of the piston to fine-tune the ride characteristic.
• Adjusts more rapidly to changing road and weight conditions than standard shock absorbers
• Two shocks into one – comfort and control

C. Twin Tube -ASD Design (Reflex )

A new twist on the comfort/ control compromise is an innovative technology which provides greater control for handling while improving ride comfort called Acceleration Sensitive Damping (ASD).
This technology moves beyond traditional velocity-sensitive damping to focus and address impact. This focus on impact is achieved by utilizing a new compression valve design. This compression valve is a mechanical closed-loop system, which opens a bypass to fluid flow around the compression valve.

Advantages:
• Control is enhanced without sacrificing driver comfort
• Valve automatically adjusts to changes in the road condition
• Reduces ride harshness

2. Mono-tube design (Standard Types)

These are high-pressure gas shocks with only one tube, the pressure tube. Inside the pressure tube, there are two pistons: a dividing piston and a working piston. The working piston and rod are very similar to the twin-tube shock design. The difference in actual application is that a mono-tube shock absorber can be mounted upside down or right side up and will work either way. In addition to its mounting flexibility, mono-tube shocks are a significant component, along with the spring, in supporting vehicle weight. Another difference you may notice is that the mono-tube shock absorber does not have a base valve. Instead, all of the control during compression and extension takes place at the piston.
During operation, the dividing piston moves up and down as the piston rod moves in and out of the shock absorber, keeping the pressure tube full all times.

Advantages:

• Can be mounted upside down, reducing the unsprung weight
• May run cooler since the working tube is exposed to the air
• Original equipment many import and performance domestic passenger cars, SUV and light truck applications

SHOCK ABSORBER/ DUMPER

SHOCK ABSORBER/ DUMPER

Twin Engine Technology

VALVE TRAIN: COMPONENTS, TYPES AND THEIR FUNCTION

The main function of the valve train, as indicated by its name, is to control the opening and closing of the valves and, for older models, the fuel output of the injectors. Most of the heavy-duty diesel engines we work with are 4 valve engines, meaning there are four valves in each cylinder: 2 intakes and 2 exhaust. The valve train uses different components based on the type, push on or lift up from the valves, allowing air into and out from the cylinder. In the middle of all the valves is the injector, which will be pushed down on to inject fuel into the cylinder. All of the timing for this process is incredibly precise. Newer engines use electrical signals to cue the injector, rather than the mechanical valve train, which makes that process even more precise.

Most new engines have overhead cam assemblies. Other designs locate the camshaft lower in the engine and use push rods to move valve assemblies. The camshaft is rotated by a timing belt, timing chain or direct gear.

VALVE TRAIN COMPONENTS

The valve train can have many components. The following are the most common components in the valve train. Depending on the type of engine, there may be varying quantities of the parts listed below or the engine may not contain all of the parts listed.

1. Camshaft
The camshaft is a long shaft that goes through the head or the block of the engine, depending on what type of engine it is. There are lobes along the length of the shaft positioned differently. The profile of the lobes has an egg-shape to them. The dimensions of these lobes are what determines the amount of lift. The more lift, the longer the valves stay open, which allows more air into the cylinder.

2. Camshaft Followers
A cam follower is a type of bearing that follows along the lobes of a camshaft as it rotates, providing a low-resistance surface for the lobe to push up against. A follower is also called a lifter, and sometimes a tappet. There are several types of cam followers, whose configurations generally depend on how they mount to their mating part. They will be used when the cam is in the block, rather than being overhead.

3. Push Rods
Pushrods are one of those parts that are not always used in a diesel engine. They will also only be used when the cam is in the block and not overhead. A push rod is a rod that pushes up on the rocker arm. It will move depending on the movement of the camshaft follower. Another job of the pushrod is to conduct oil up to the cylinder head.

4. Rocker Arms
A rocker arm is a pivoting lever that pushes on the valve stem. Rocker arms will sometimes be called rocker levers, or just rockers. Depending on the type of valve train, the rotating camshaft lobes will either push directly on the rocker arm, or on the pushrods, which will conduct the motion up to the rocker arm. In an overhead cam engine, the cam follower is built into the rocker arm in the form of a roller.

5. Rocker Shafts
Rocker shafts are simply the shafts that the rockers are on. It’s this shaft that is the pivot point for the rocker arms. The shaft also conducts oil to the various rocker arms.

6. Valve Bridges
Valve bridges are also sometimes called valve yokes. Bridges allow a single rocker to actuate multiple valves. It has a stem or bridge that sits on both valve stems, so that when the rocker is pressed down, the valve stems get pressed down as well.

7. Valves
A valve is composed of two major sections, the valve head, and the valve stem. The head of the valve is what allows air into and out of the cylinder. The stem is what gets pressed on by the rest of the valve train. At the end of the stem are grooves that keepers will fit into to hold the valve in place. Some engines have only two valves per cylinder, and some have four. The more common number in the heavy-duty diesel market is four. These are split evenly between the intake and exhaust valves.

8. Valve Springs
The camshaft creates an upward force that acts on the rocker arm, which in turn pushes the valve down. But as the cam rotates around, it does not pull the pushrod or rocker arm back with it. That’s why there is a valve spring to create force in the opposite direction and close the valve. The spring will hold the valve closed until the lobe of the camshaft comes around with a greater force and pushes it down.

9. Timing Belt:
A timing belt instead of a timing chain may be used to turn the camshafts. The inner side of the belt is designed with square (cogged) teeth which prevent the belt from slipping.

10. Belt Tensioner
The belt tensioner is a spring-loaded wheel which keeps the timing belt in tension and aligned with the cam sprocket. The smooth side of the timing belt rides over the tensioner. The tensioner applies a force on the backside of the belt. This keeps the belt in tension. Whenever the belt needs to be removed, the tensioner can be pulled away, freeing the belt.

TYPES OF VALVE TRAINS

1. OHV or Push-rod valve train

In case of OHV or push-rod systems, there are long rods which have to be pushed by the camshaft lobes to move the valve rockers, which in turn open the valves – thus the name ‘push-rod’. The long rods and the mechanical nature of the pushrod system make it heavy and it’s not compatible with engines which run at higher revolutions per minute. Now while OHV is an older design, it has its advantages in terms of simplicity of design, compact packaging and a simpler lubrication system requirement as compared to an OHC system.
The disadvantages, of a pushrod system, however, are many.
• To start with, the engines with an OHV system cannot run very high RPMs and such valve trains are suitable mostly for low engine speed applications such as heavy cruisers.
• Owing to the heavy components, the noise and friction on such systems are much more than an OHC system.
• Also, any issues with the camshaft require the entire engine to be opened up, as the camshaft sits inside the engine block, which increases the maintenance effort and cost in case of a breakdown.
• Finally, OHV engines lend their design well primarily to two-valves per cylinder layout. It’s not that there aren’t any three or four valves per cylinder engines with OHV, but that setup becomes way more complex, and OHC systems offer much more flexibility with multiple valves per cylinders.

2. OHC Valve trains

To overcome the shortcomings of the pushrod valve trains, OHC valve train was developed. As the name suggests, it’s a valve train configuration where the camshaft for the engine is placed over the head of the engine, above the pistons and valves. This design allows for very direct contact between the camshaft lobes and the valves or a lifter, thus reducing mass, reducing components and allowing better engine performance as well as more flexibility with the overall engine design.

A. Single Overhead Cam/SOHC

For this variety of valve trains, there is a single camshaft for each row of engine heads. So a single cylinder OHC engine will have one camshaft. However, if it’s an engine with multiple rows, say a V6, then it will have two camshafts – one for each row of heads, or each bank. For SOHC engines, the camshaft is connected directly to the crankshaft via a timing belt or chain to ensure that the opening and closing of the valves is perfectly in sync with the various strokes of the engine for each cylinder.

Now, with SOHC, there is an option to either open or close the valves directly with a shim between the cam lobe and the valve stem, or via a rocker arm. Valves have springs which return them back to their closed position once the pressure from the camshaft lobe is off. SOHC engines are also suited better for 2 or 3 valves per cylinder configuration. Not that a SOHC valve train cannot run on a 4 valve per cylinder layout, but the whole set-up then becomes too complex for the design of rocker arms and lobes and it’s generally considered better to employ a DOHC valve train is such scenarios.

B. Double Overhead Cam/DOHC

DOHC or dual overhead camshaft design includes two camshafts for every row of cylinder heads. Talking about the example we took for SOHC, a DOHC setup for a single-cylinder engine will have two camshafts. However, if it’s a V6, it will have 4 camshafts, two for each row of engine heads, or banks. The primary advantage of such a setup is that it allows manufacturers to have a well-engineered answer to handling a 4-valves per cylinder. Generally, one of the camshafts handles the intake valves, while the second one handles the exhaust valves. The 4-valve per cylinder setup allows for better breathing for the engine, and better performance in most cases, making DOHC a choice for engines that need to rev higher. A DOHC setup also allows for putting the spark plug bang in the middle of the cylinder head, which facilitates better combustion, and enhances performance, and fuel efficiency of the engine. With SOHC, such a setup is not possible for 4-valves per head, as it has to sit in the middle of the cylinder head so as to operate both intake and exhaust valves. As mentioned before, though, SOHC engines too can handle four valves per cylinder, and while the construction of such valve trains is complex, it’s desirable in some cases. DOHC brings along the extra weight of the additional cam, though by allowing the positioning of the spark plug in the middle of the cylinder head it also enhances optimum combustion of fuel. In a nutshell, DOHC is more suited for high-performance engines which need to rev higher and perform in the higher rev range. SOHC systems have somewhat better lower end torque though.

Finally, a DOHC system, with its more fine-grained control over valves is more suitable to implement variable valve timing for engines. Such systems utilize variable camshaft profiles for different engine speeds to enhance performance across the entire rev band. The control over the speed and position of valves opening and closing is better in case of DOHC, and in today's electronics driven world, great benefits can be extracted using that fact. DOHC valve train is more expensive than SOHV though and coupled with its suitability for 4 valves per cylinder, it makes it feasible to employ that setup only on automobiles above a certain price point. For applications where everyday usability, low and mid-range torque, simplicity of design, easy construction and cost are important factors, SOHC system works well.

Valve Train