Spark Plugs

Spark plug is a device used commonly in Spark Ignition (S.I) engines to ignite the air-fuel mixture in the combustion chamber. It produces electric spark when current is passed by the ignition system.

Gottlob Honold of Bosch developed a spark plug that helped in the development of spark ignition engines in 1902. Later in 1903, Oliver Lodge developed a more reliable version of spark plug to fit in S.I engines.

When does a spark plug fire?

We all are aware of the four strokes of S.I engines. At the end of compression stroke, the pressure build up in the combustion chamber becomes intense. The fuel breaks into finer particles due to the compression of charge. This is the easiest time to ignite the fuel and hence spark plug is excited by the ignition system to generate an electric spark that generates enough temperature to ignite the air-fuel mixture.

Spark plug is installed at the top of the cylinder head and ignites when the piston reaches top dead centre (TDC) in the compression stroke.

Functions of a Spark Plug:

A Spark plug basically has two main functions:

·         To ignite the air-fuel mixture. The electric energy passes through the central electrode and jumps the gap at the tip to generate spark, which then ignites the air-fuel mixture.

·         To dissipate the heat from combustion chamber. We might be at a misconception that spark plug creates heat, but no. Spark plugs act as a heat exchanger remove the heat from combustion chamber and transferring it to the cooling system. The temperature at the tip should be maintained such that it should not be high enough to lead to pre-ignition and it should not be low enough to result in fouling.

CONSTRUCTION OF A SPARK PLUG:

Insulator Body:

Insulator body is made of aluminium oxide ceramic. The shape of the body is moulded in a dry moulding system. After it is dry moulded, it is heated to a temperature higher than the melting point of steel. This provides high thermal conductivity and shock resistance. The surface is ribbed for better grip and protection from secondary voltage.

Terminal:

It is also known as connector. This is where the spark plug wire from the ignition coil is connected to the central electrode.

Hex Head:

This part is to fit the socket wrench in order to tighten and loosen the spark plug in its hole in the cylinder head.

Shell:

Shell is made of steel and is fabricated with the help of cold extrusion process. The shell is plated for corrosion resistance.

Gasket:

Gasket is used for sealing purpose. Threads are provided to screw the spark plug in the cylinder head.

Central Electrode:

The central electrode is connected to the outer terminal. It is an internal wire that conducts current from outer terminal to its tip. The tip is usually made of copper to carry away the heat of combustion.

Ground or Side Electrode:

They are manufactured from nickel alloy steel. They have an edge shaped area at the bottom to create the gap between central and side electrode. The high voltage jumps from the central electrode to the side electrode to create the required spark.

Spark Plug Working:

The high voltage required to generate a spark is supplied by an ignition coil. As the electrical energy passes through the central electrode, a voltage difference is created between the central electrode and the ground electrode. We all know that spark plug is fitted at the top of the cylinder head. Hence, air-fuel mixture will be present in the gap between the two electrodes.

Air-fuel mixture acts as an insulator, therefore no current can flow between the electrodes. Once the voltage is high enough to exceed the dielectric strength of the air-fuel mixture, the mixture is ionized. Ionized gases act as a conductor and allow the electrons to flow through them, thereby creating a spark. Different spark plugs require different voltages to generate the spark. It usually ranges between 20,000 V to 1,00,000 V.

A small flame is created at the tip of the spark plug which then flows as a flame front at the top of the piston depending on the composition of air-fuel mixture.

Types of Spark Plugs Based on Temperature:

Spark plugs can be divided into two types: Cold plug and Hot plug.

·         Cold Plug: Cold plugs are used in engines that generate high horse power and high compression pressure and temperature. It has less insulation; therefore it can transfer more heat from combustion chamber to the cooling system. It is of prime importance to transfer the excess heat from the combustion chamber to the outside in order to prevent pre-ignition and engine damage.

·         Hot Plug: Hot plugs have more insulation compared to cold plugs. These plugs are available in most standard engines. These spark plugs retain more heat to burn off the excess carbon deposits.

What is Spark Plug Fouling?

Fouling is known as coating of the insulator tip with foreign particles such as oil, carbon or fuel. This allows the high voltage to leach back down to the metal shell and to the ground instead of jumping the electrodes gap and creating spark.

  

Homogeneous Charge Compression Ignition (HCCI) engine

We have always heard about two types of engines which dominate the automotive market when it comes to internal combustion engines, namely: Spark Ignition (S.I) and Compression Ignition (C.I). Homogeneous Charge Compression Ignition (HCCI) engine is a blend of both S.I and C.I engines.

We all know that air-fuel mixture in a S.I engine is a homogeneous charge (i.e.) air and gasoline easily mix together at room temperature and require a spark plug to be ignited. Diesel on the other hand is less volatile and C.I engine can combust fuel as soon as it reaches the auto ignition temperature without the help of spark plugs. HCCI engines burn gasoline under the influence of high temperature and pressure during compression. Therefore, HCCI provides us with the best of both worlds.

Working of HCCI:

The working of HCCI is based on the four stroke Otto cycle. During the suction stroke, gasoline and air mixture is introduced in the combustion chamber. It is important to keep the air-fuel mixture much leaner in order to help in effective combustion process to take place. A conventional S.I engine has a stoichiometric air-fuel ratio of 14.7:1. But in a HCCI engine, the air-fuel mixture ranges from 24:1 to 31:1 which makes it easier for the engine to self ignite the charge.

Next follows the compression stroke. Unlike a conventional S.I engine, HCCI engines usually have a higher compression ratio up to 16:1. This helps in achieving the required temperature and pressure to auto ignite gasoline.

Just like a conventional C.I engine, the combustion takes place right at the end of compression stroke in a HCCI engine. The piston then moves from T.D.C to B.D.C to complete power stroke. Exhaust stroke follows the power stroke. The emissions are much lesser compared to both S.I and C.I engines.

Advantages of HCCI:

·         Fuel consumption can be reduced by 15% compared to S.I engine.

·         Emissions are lower due to lower peak temperature.

·         It can run on gasoline, diesel and other alternative fuels.

Disadvantages of HCCI:

·         Higher cylinder pressure can damage the cylinder.

·         Auto ignition can be difficult to control. It can lead to excessive knocking.

·         Power generated is less compared to S.I engines.

·         Cold starting is difficult without a spark plug.

Engine Muffler

Why do Engines make noise?

You would have seen lots of vehicles on road that make different noises. An engine makes a lot of pulsating noise as the exhaust gas escapes the exhaust valves at a very high pressure. These sounds bounce around the inner walls of the tail pipe and can create a loud and annoying noise. A muffler is used to minimize the sound and also tune the sound before the exhaust exits the tail pipe.

Where is a Muffler installed?

Mufflers are installed usually at the end of tail pipe. You can actually spot it as a big box and it does not treat the pollutants in the exhaust gases. It acts as an acoustic soundproofing device designed to reduce the loudness of the engine noise.

How does a Muffler work?

Mufflers are lined with baffles. As the exhaust enters the muffler, the sound waves bounce off these baffles, thereby creating opposing sound waves that cancel each other out. The baffles and chambers can also be tuned to get the desired sonic effect. We can either cut the sound as much as possible or focus on the desired sound with amplified growl range.

Can a Muffler affect engine performance?

Yes, a muffler can affect engine performance. The engine requires fresh charge as soon as the exhaust escapes the exhaust valves. The faster we can get rid of the exhaust from the exhaust pipe, the faster we can supply fresh charge to the engine and can improve its performance. Installation of muffler shouldn’t affect the flow rate of the exhaust from the system.

Kinetic Energy Recovery System (KERS)

Kinetic Energy Recovery System (KERS) is commonly used in Formula 1 cars as a device for recovering kinetic energy in the form of electrical energy when brakes are applied. In addition to the 1.6 liter V6 engine which can produce 600 hp, KERS can provide an additional 160 hp which can boost the speed of the car to overtake or to increase the lead from the other cars.

Why is KERS required?

The introduction of the new KERS system has significantly reduced the size of the Formula 1 engines from 2.4 liter V8 to 1.6 liter V6. This has resulted in higher efficiency and reduced the fuel consumption by approximately 35%.

FIA allowed the use of KERS in Formula 1 cars for the first time in 2009. At that time, the system could only produce 80 bhp and was also not mandatory for the teams to install in their cars due to space and weight constriction.

However in 2014, all companies agreed to install the new KERS which could generate 160 bhp and also to balance the shift from 2.4 liter V8 to 1.6 liter V6 engine.

Working of KERS:

The KERS introduced in 2009 was used to draw energy from rear axle of the car and could store 400 KJ of energy per lap, which can be reused in the form of boost to add 80 bhp to the wheels. The energy is recovered during braking and is reused as a boost for 6.6 seconds per lap. The recovered energy can be stored electrically, in a battery or a supercapacitor or mechanically, in a flywheel. The energy can be recovered by the driver by using a ‘Boost button’ on the steering wheel.

The new system introduced in 2014 allowed the system to recover energy from all four wheels and also from the exhaust gas. It consists of two separate Motor Generator Units (MGUs). The MGUs in the form of generator generates electricity and it can also function as a motor generating mechanical energy.

MGU-K (where ‘K’ stands for kinetic energy) can convert the kinetic energy generated during braking into electrical energy and store it in the supercapacitor. It also acts as a motor to power the drivetrain by returning approximately 160 hp. This unit can recover a maximum of 2 MJ of energy.

MGU-H (where ‘H’ stands for heat energy) can convert the heat energy from the exhaust into electrical energy. This unit is connected to the turbocharger and also controls the speed of the turbocharger. The energy recovered by this unit is used to power the MGU-K unit and further transmitted to the drivetrain. The maximum amount of energy that can be recovered by both the MGUs is 4 MJ, which is 10 times higher than the 2009 model.

The driver can use an additional boost of 160 hp from KERS for 33 seconds per lap.     

The power unit consists of 6 components:

·         Internal combustion engine

·         Motor Generator Unit (MGU-K)

·         Motor Generator Unit (MGU-H)

·         Energy Store ( Supercapacitor)

·         Turbocharger

·         Control Electronics

The teams are allowed to use only 5 of the 6 power units during the entire championship season. If they use all 6, grid penalty will be imposed on the driver. 

How to Calculate Vehicle Speed

Car transmissions can look complicated, and the actual working can seem to be even more complicated. A conventional constant mesh gear box consists of an input shaft from the engine, a counter shaft and a main shaft which delivers power to the differential via propeller shaft. To know more in detail about the working of a constant mesh gearbox, please visit the page on the following link: 

• How manual transmission works

As you can see in the diagram, a 4-speed constant mesh gearbox consists of a set of 11 gears (including the gears between the input shaft and the counter shaft). Now let’s calculate the gear ratio between the gears and how a gear ratio can affect the final drive given to the wheels? How to calculate the speed of the vehicle?

1^st gear:

Let’s say the gear A (driving) has 10 teeth and gear B (driven) has 35 teeth. Gear ratio is the ratio of number of teeth in the driven gear to the number of teeth in the driving gear.

Gear ratio = number of teeth in the driven gear / number of teeth in the driving gear

Therefore, 1^st gear ratio can be calculated as

G1 = T_B / T_A

G1 = 35/10 = 3.5 : 1

T_B = Number of teeth in gear B

T_A = Number of teeth in gear A

The differential has its own gear ratio which is known as the Differential gear ratio (G_D). In this case, let’s assume that G_D = 3.5. Now the G_D is fixed and cannot be altered.

Now to calculate the speed at which the wheels are rotating, we need to bring into picture the final gear ratio. Final gear ratio decides at what speed the wheels are driven. It is a product of both transmission gear ratio and the differential gear ratio together.

Final Gear Ratio (G_F) = G1 X G_D

G_F = 3.5 X 3.5 = 12.25

Yes, I know it is very complicated and you are lost somewhere in understanding the whole concept. To explain you in simple words, the final value of G_F = 12.25 indicates that for 12.25 revolutions of the engine crankshaft, the wheels will revolute only once.

Consider your engine running at 2000 rpm, then wheels will rotate at (2000/12.25) rpm.

To calculate the speed of the vehicle:

Let’s consider the tire is 0.35 m in diameter, therefore the circumference of the tire is 

C = πD

C = π(0.35)

C = 1.1 m (approx.)

Hence, for every 12.25 revolutions of the crankshaft, the wheels will cover 1.1 m.

Now let’s consider the engine speed in revolution per hour (rph) = 2000 x 60 = 1,20,000 rph.      

Now the vehicle speed can be calculated using the above values. The vehicle speed at 1^st gear at an engine speed of 1,20,000 rph is

Vehicle speed = (Engine speed in rph / final gear ratio) X circumference of the tire

Vehicle Speed = (1,20,000 rph / 12.25) X 1.1 m

= 10,775 meters per hour (approx.)

= 10.775 km/h

The vehicle speed for the other gear ratios can be calculated by following the same procedure as above. Let’s calculate:

2^nd Gear:

Let’s assume the 2^nd gear ratio, G2 = 2.5 : 1.

Differential gear ratio (G_D) = 3.5 : 1

Final gear ratio = 2.5 X 3.5 = 8.75 : 1

Engine speed = 200000 rph          

Vehicle speed at 2^nd gear = (200000/8.75) X 1.1

 = 25142.85 m/h 

= 25 km/h (approx.)

3^rd gear:

Let’s assume the 3^rd gear ratio, G2 = 1.8 : 1.

Differential gear ratio (G_D) = 3.5 : 1

Final gear ratio = 1.8 X 3.5 = 6.3 : 1

Engine speed = 200000 rph          

Vehicle speed at 3^rd gear = (200000/6.3) X 1.1 

= 34920.63 m/h 

= 35 km/h (approx.)

4th Gear:

Let’s assume the 4^th gear ratio, G2 = 1 : 1.

Differential gear ratio (G_D) = 3.5 : 1

Final gear ratio = 1 X 3.5 = 3.5 : 1

Engine speed = 200000 rph          

Vehicle speed at 4^th gear = (200000/3.5) X 1.1 

= 62857.14 m/h 

= 63 km/h (approx.)

Shock Absorber

Shock absorber (also known as damper) is a hydraulic device used commonly in automobiles to absorb and dampen the vibrations which a vehicle experiences while traveling on irregular surfaces. Kinetic energy is converted into heat energy and the heat is dissipated.

A shock absorber is usually used in conjunction with coil springs. It actually absorbs the natural frequency of the spring and dampens the unwanted spring motion. Imagine a car with only springs. The drive will be a lot bouncy without shock absorbers.

Design of a Shock Absorber:

Twin tube shock absorber is the most commonly used type of shock absorber. So let’s discuss the working of a twin tube shock absorber. As the name suggests, it consists of two tubes. The outer tube, also known as reserve tube mount is connected to the frame of the vehicle, whereas the inner tube (also known as pressure tube) is connected to the wheel axle.

The reserve tube is internally connected to a piston via a piston rod. The reserve tube with a piston sits on the pressure tube filled with hydraulic fluid. Orifices of very small diameters are pocketed out in the piston.

Working of a Shock Absorber:

When a vehicle hits a bump, all the kinetic energy from the spring is transferred to the reserve tube. The kinetic energy is then transferred to the piston. To explain in a simple way, the reserve tube exerts a lot of force on the pressure tube. The hydraulic fluid in the pressure tube is compressed and a small amount of fluid escapes through the tiny orifices in the piston under a great pressure. Since only a small amount of fluid escapes to the reserve tube, the piston moves down slowly and hence the spring movement is dampened.   

Regenerative Braking System

Every time we hit the brakes on a moving car, all the energy that the engine has supplied to the wheels goes to waste. In other words, the kinetic energy that is used to power the wheels is converted into heat when brakes are applied. The heat generated is of no use and thus we waste valuable energy.

Engineers have come up with a method to use the braking energy to be converted into electrical energy and recharge the battery. This method of utilizing the braking energy to be converted into electrical energy is known as Regenerative Braking. This system captures most of the kinetic energy and converts into electricity, which can recharge the battery. This system is used more commonly in hybrid and electric cars because it is imperative to recharge the batteries to make the car run longer.

How does braking waste energy?

Any normal guy driving a car would think that he is not wasting much energy in applying the brakes, because he is just using his foot to press the pedal. It doesn’t require a huge amount of force to apply pressure on the brake pedal and bring the car to a stop. So how are we wasting energy while braking?

Look at the above scenario from an engine’s perspective. Engine does all the work to generate power and supply it in the form of kinetic energy to the wheels. A lot of energy in the form of fuel is used to provide the momentum to the wheels. When brakes are applied, the car’s momentum is lost and it either slows down or comes to a halt. All the kinetic energy is wasted in the form of heat due to the friction between the brake pads and the drum. Now to start the car again from the start requires more fuel to provide the momentum. Hence, we are wasting a lot of valuable energy while braking.

Regenerative Braking System:

As already mentioned, regenerative braking technology is used in hybrid and electric cars because the wheels of these cars are powered by electric motors. The electric motors convert the electrical energy from the battery into kinetic energy which drives the wheels. During braking action, instead of using brake pads, the electric motor starts rotating in reverse direction.

When the driver steps on the brake pedal, the electric motor switches to generator mode by reversing its direction of rotation. Through its rotation, the generator converts a portion of the kinetic energy into electrical energy. This electrical energy is then stored in a high voltage battery. As a result of this electricity generation, generative braking torque is produced by the motor which decelerates the wheels. However, the generative braking torque won’t be sufficient to completely halt the vehicle. Therefore, once the vehicle is decelerated to a low speed, conventional friction brakes (disc or drum brake) further assist in stopping the wheels.

In the case of emergency where vehicle has to be stopped immediately, only friction brakes will be used. Regenerative brakes are more useful in stopping a slow moving vehicle.

Advantages of Regenerative Braking:

·         It enables an extended battery range in electric vehicles.

·         Reduces fuel consumption and CO₂ emissions in hybrid vehicles.

         

 Related Topics:

• Disc Brakes

• Drum Brakes

• Anti-lock Braking System (ABS)

• Traction Control System (TCS)

• Electronic Brake force Distribution (EBD) 

• Air Brakes