Engine Tech

Understanding Engineering Terms 

Publishers will give descriptions of engines and vehicles and they assume that there is a level of understanding, that the reader will know what is being talked about.  This is not always true and when delving deeper into engineering, it is always advisable to familiarise yourself with as much as possible.  This page is going to walk you through terms that are regularly used in detail.

The Engine 

One skill an engineer needs is to be able to accurately measure and understand measurements.  The components inside an engine are so finely tuned to work together, miss matched tolerances can cause failure.  Most cars have more than one cylinder (four, six and eight cylinders are common). In a multi-cylinder engine, the cylinders usually are arranged in one of three ways: inline, V or flat (also known as horizontally opposed or boxer).

Induction

The inlet valve is opened and the movement of the piston from TDC to BDC creates a vacuum that draws the supplied air and fuel mixture into the cylinder.

Compression

Both valves are closed and the piston moves from BDC to TDC reducing the volume in the cylinder.  This combination of mixing the air together better and compressing and increasing the temperature in the air allows for more of the available energy in the air:fuel mix to be utilised.

Power

Ignition occurs before the piston reaches TDC in most engines, the resulting expansion of the air created by heat from the combustion works against the piston and forces it down thus turning the crankshaft.  The change from chemical energy to heat energy then mechanical energy is the creating the power stroke.

Exhaust

The cylinder is cleared of the remaining burnt gas in the exhaust stroke that has remained in the cylinder after the initial gas has been expelled as the pressure is released upon opening of the exhaust valve.  The piston upwards motion sweeps the remainder out in readiness for fresh air refill. 

The Components of the Engine

Camshafts

Camshafts, the component that moves the valves, are shaped depending on the amount of time the valve is required to be open.  The camshaft rotates at half the speed to the crank so the power stroke only see’s one 4th of the turn of a camshaft or 90°

The movement of the valve from the cam can be measured in a graph, this is used in timing of the engine.

Cylinder

The cylinder of the engine is measured and developed perform many tasks.  The main one being combustion of fuel.  It is also designed to create turbulence for the air as it enters the chamber.  The cylinder is classified by the diameter (bore) and depends on how many cylinders to the size of the engine.  

Valves

The valves are what control the flow of air through to the cylinder.  They are moved by the rotation of the cam lobes and can be altered in movement to change the amount of air needed to fully combust the fuel.  They are designed to influence the turbulence of air and measured in stem length and head diameter.

The weight of the moving parts in the engine are important, this is why valves have a thin stem and narrow diameter head, to have a bigger valve the material would have to be looked at.  Titanium is used as it has a density lighter than steel yet is very hard.  This means that the high RPM of the engine can be reached without much wear on the moving components.

The region around the valves where the air passes through is the ‘curtain’ area.  This is the valve circumference x valve lift.  This means that the curtain area will increase as valves open.  The valve will no longer represent the limiting factor when the valve lift is 0-25 the diameter.  This  will mean that the curtain area is equalling the valve area.  If a valve is flowing perfectly this is the point where 100% efficiency and after this more valve lift would (theoretically) be pointless.  But because no flow is 100% efficient higher valve lift beyond this point is needed.

Piston Head

The piston head is the component of the engine that most people recognise.  It is designed to withstand both high level forces and temperatures and transfers the force from heat energy to work done. There are many designs of piston, ranging from flat topped to indented.  These are designed to produce the maximum pressure and efficiency.  The piston head is shaped to cater for the piston rings as the diameter of the piston is not identical to the bore of the cylinder.

Conrod

This is the component that transfers the force from the piston through to the crank.  The stresses that are imposed on the rod are high and the material that the rod is made from is very important.  Lots of testing goes into the profile of the rod to make sure that it does not fail.  The piston head is attached to the conrod via the…..

Gudgeon Pin.

This pin is a push fit part that needs to withstand the transfer of the force created in combustion from linear to rotational force.  

Crankshaft
This component is worked on by the piston and con-rod.  It is the strongest component in the engine as the forces that are exerted onto it are transformed from linear to rotational and the crank harnesses the power to move the vehicle.

The Cylinder Head

The cylinder heads main priority is to seal the top of the cylinder so that only the required air can flow into the engine.  They are made of either cast iron (older engines) or an aluminium alloy.  The gas pressure and temperature that is exerted on the head means that it is constructed as a rigid solid body.  When designed, the cooling of the engine needs to be considered, whether it is spark ignition and the position of the valvetrain.  Most modern engines are overhead valve designs which improves volumetric efficiency.  Cylinder heads are either single (SOHC) or double overhead design (DOHC) and may operate the valves either directly or indirectly.

Inline OHV ~ valves are generally arranged vertically, with bathtub heart or kidney shaped combustion chambers.

Heron head ~ OHV engine with inline (vertical) valves and the combustion chamber in the piston.

Wedge ~ the valves are inline but angled from the vertical creating a wedge-shaped combustion chamber.

Pent-roof ~ the valve heads are opposed (stems in a V formation) in a sloping sided combustion chamber.  Increasingly used for modern four valve per cylinder layouts.

Hemi ~ the valve heads are opposed but the chamber is hemispherical.

L-type or flathead ~ an older design used for engines contained within the cylinder block. (sidevalves) 

Displacement and Clearance Volume

The combustion chamber is the area where compression and combustion take place. As the piston moves up and down, you can see that the size of the combustion chamber changes. It has a maximum volume as well as a minimum volume. The difference between the maximum and minimum is called the displacement and is measured in litres or CCs (Cubic Centimetres, where 1,000 cubic centimetres equals a litre).

The area left in the cylinder at TDC is the clearance Volume, this is where the air is compressed ready for ignition.  This area is sometimes increased with indentations on the piston head, these indentations are also used not to increase but to decrease the clearance by providing room for the valves to move into the cylinder without hitting the piston head.  The smaller the area that the air is compressed to the bigger the force on the piston.

To calculate the displacement without knowing the overall size of the engine

Examples:

*A chainsaw might have a 40 cc engine.

*A motorcycle might have a 750 cc engine. If it is a 4 cylinder engine then each cylinder will have    187.5cc in each one

*A sports car might have a 5.0 litre (5,000 cc) engine.  A V8 would have 8 625cc cylinders.

Torque

Is the turning effort about the crank shafts axis of rotation. 

torque=force x radius    [N]

Torque generically is the moment of a force about a given point that is the product of the force and the perpendicular distance from the line of action of the force to that point.  The perpendicular distance is the leverage of the force.    When the moment has a tendency to twist or rotate a body, such as the crank, it is the turning moment or torque.

Brake power

Most people are used to hearing about horse power when referring to vehicles and comparing the power output.  The horse power is a relative amount, Watt worked out that one horse can do 33,000 foot-pounds of work every minute.  1 horse power is equivalent to 746 watts.  

The brake power (b.p) of an engine is the useful power available at the crankshaft of the engine. It is measured by running the engine against some form of absorption brake, hence its name.

If       S= spring balance reading (N)

             W1=static load (N)

               W= effective load on torque arm (N)

                      R=length of torque arm (m)

Torque τ transmitted by the engine = Effective load x length of torque arm

τ = WR Nm where W=S+W1

If the engine is running at N rev/min then:

Power/torque curves show the relationship between the two.  The bigger the distance between the two peeks the more usable the engine is. 

Because R is a given fixed length for a given dynamometer so the term 2πR/60 may be written 1/K

Indicated Power

Indicated power is the actual maximum power developed in the cylinder.  It assumes there are no losses through the cycle and is always higher than b.p.  This is due to the losses outside the cylinder that affect b.p.

Mechanical Efficiency

The ratio of the useful power available at the output shaft to the power developed in the cylinders of an engine is known as the mechanical efficiency.

Normally the efficiency is rated as a percentage.

The mechanical efficiency is reliant on the engine speed, air intake temperature and degree of carburettor throttling.  In general the efficiency increases with engine speed and/or with higher jacket water temperatures. 

Brake Mean Effective Pressure

When the mean effective pressure is based on the brake power of an engine it is known as the break mean effective pressure (b.m.e.p) and is obtained by

The b.m.e.p is proportional to the engine torque (or load) and if the suitable scale is used the b.m.e.p scale will match the torque curve.

Can also be written as:

Where A = area of piston crown (m2)

                L = length of stroke (m)

                n = number of working strokes/min

The difference between the indicated power and the brake power of an engine is termed the friction power 

Fuel Consumption

When it comes to selling vehicles, especially cars, the fuel consumption of the engine is very important and manufacturers will give figures created through tests run in a clean environment.  Consumers will measure fuel consumption in miles per gallon but engineers will measure fuel consumption in kilograms or litres maintaining 1 kW of indicated power for a period of one hour. 

Volumetric Efficiency

This is the efficiency with which the engine manages to bring air through the system to the ports and uses it during combustion.

Thermal Efficiency

During the combustion cycle an engine cannot convert all the heat energy in the fuel into useful work.  This is because the energy is converted into many other energy forms.  The most thermally efficient engines will still only convert around a third of the available energy into work. 

The calorific value is measured in joules per kg

Air Flow

It is always better to get cold air into the chamber then heat through compression as hot air is less dense than cold.  This makes the engine more efficient.  The air needs to be hotter than ambient at combustion for it to take place efficiently so heat increase in compression needs to be sufficient.  Air too cold at combustion and the fuel will not burn efficiently.

The burn rate had to be faster than the burn stroke.

i.e.   Stroke = 80mm

Speed = 1/200 s

Burn rate needs to be a minimum of

80/(1/200)   mm/s

=16000mm/s

= 16m/s

= 35mph

The higher the chamber pressure is built the better the fuel consumption.  Injecting the fuel before the valves cools the air before it enters the chamber, fuel needs energy from the air to atomise.  

Using turbo chargers brings the compression ratios down.

When the engine is 'choking' it's not getting enough air.  Air can not flow faster than the speed of sound. 330m/s

To avoid spring surge in valves there are two springs with different frequencies. They touch so that they cancel each other out. Sometimes an object that isn’t a spring is used.

Fc – spring force = moving mass x acceleration

Fc – spring force = contact force – spring force

Therefore

Net downforce 

Fc = mass x acceleration + spring force

Piston Friction

As pressure pushes down the rod resists and pushes the piston to the side as the force travels along the rod.  Wear on a cylinder is on the ‘thrust’ side which is the side the rod will face on the down pressure stroke.

To resolve vertically

Fgas  =  Frod . Cos Ø

Horizontally

Fgas = Frod . SinØ

Sound travel

Sound travels with a pressure wave, molecules closer to the source of the noise move towards an outward layer increasing the pressure on the second layer.

Newtonian law of F=ma means then the second layer start to move on towards the third layer.  The force is smaller each time and eventually the wave fails.

Different size molecules alter the distance the noise travels.  Gas such as small molecules helium men sound travels far through it.  Now think of the sound as air travels through the engine.

Fast molecules entering the tube pressurise the stationary molecules because the wave in front of them travels at the speed of sound. As long as the fast molecules are traveling slower than the speed of sound the pressure wave moves the stationary molecules ready

As RPM goes up, piston speed goes up, air speed goes up in the port.  When the air in the port reaches the speed of sound the revs cannot get faster without choking.

The air flow is disrupted so the effective flow area is smaller than expected.  The throat of the port is where turbulence is created.

Therefore effective throat area at full lift:

= aprox 0.5x actual area

F1 engines with straight inlets = 70% of actual area at full lift.