Powertrains in Motorsport

In this article I will look into the aim of a powertrain, sources of power and prime movers, the four stroke engine cycle, mean effective pressure, calculating the indicated power of an engine, air to fuel ratios for different fuel, volumetric efficiency, ideal spark advance combustion, energy storage for motorsport vehicles, battery vs capacitor for storing energy, electric motors such as the brush DC motor and the permanent magnet DC (PMDC) and advanced ERSs in F1. Please feel free to skip to the part most relevant to you.

What is the Aim of the Powertrain?

In essence, the powertrain is the source of power which propels the vehicle forwards. Therefore, the main aim of a powertrain is to produce as much power as possible in as little mass. 
For some Motorsports, such as Formula One, powertrains also have to take into condensation fuel efficiency, keeping in touch with the problems humans face with climate change. Therefore, it can be seen that the main objective is to produce as much power as possible with minor objectives such as weight, efficiency and so on.

Sources of Power: Prime Movers

There are two main sources of power for vehicles in motorsport:
  • The heat engine
  • The electric motor
Each one of these prime movers can be split up into many different types too:
  • External combustion engine -  These are engines such as the steam engine, steam turbine, hot-air engine and closed-cycle gas turbine.
  • Internal combustion engine, rotary - These are engines such as the open-cycle gas turbine, jet engine, wankel or rotary engine.
  • Internal combustion engine, thrust type - These are engines such as the rocket, ram jet and pulse jet.
  • Internal combustion engine, reciprocating - These are engines such as the compression ignition engine, spark ignition engine and HCCI (Homogeneous Charge Compression Ignition) engine.
For motorsport purposes, the spark ignition engine is the choice of powertrain. With this type of engine with pistons, there are three types of engines:
  • Gasoline SI - This uses gasoline that enters the combustion chamber as a fuel-air mixture and uses a spark to produce the ignition of the mixture.
  • Diesel - This uses diesel to simply compress the fuel-air mixture in the combustion chamber to very high pressures: enough for the diesel to self ignite without the need of a spark.
  • Gasoline HCCI - In such engines, the mixture is mixed a lot better than traditional Gasoline SI and burns at a much lower temperature, producing much less emissions.

Four Stroke Engine

The four stroke engine consists of the following sequence of events:
  • Intake - This is where the fuel and air is injected into the combustion chamber ready to be ignited. The fuel-air mixture needs to be well mixed for the maximum power and efficiency.
  • Compression - Upon another stroke from another piston connected to the crankshaft, the piston moves up towards the fuel-air mixture, compressing it so that the pressure inside the combustion chamber increases to hundreds of psi. 
  • Expansion - Once the fuel-air mixture is compressed, the spark will ignite, causing the fuel-air mixture to burn rapidly. The increase in energy causes a huge force to be exerted on the piston, forcing it down expanding the combustion chamber.
  • Exhaust - Upon full expansion, the piston will move back up to let the used fuel-air mixture to exit out of the exhaust. The process then repeats again
It is important to remember that only one of the stages of a four stroke engine produces the power to move a car along.

Mean Effective Pressure

The mean effective pressure is the area underneath the graph of a four stroke engine with pressure on the y axis and the stages of the sequence on the x axis divided by 720 degrees:

Area under the graph is the mean effective pressure
This can be calculated as: MEP = A(under) / 720 degrees. The 720 degrees is rotation of the crankshaft which will rotate twice around per stroke (360*2 = 720). The mean effective pressure is a product of:
  • Thermal conversion efficiency - This is influenced by the air to fuel ratio.
  • Volumetric efficiency - This is influenced by the variable valve timing.
  • Mechanical efficiency - This is influenced by the ignition timing.
As well as the mean effective pressure, there is also the terminology 'peak firing pressure', which is the maximum pressure the engine reaches at which the spark can ignite.

Indicated Power

To work out the power of an engine, you can use the following equation:

P = (2*MEP*Vd*n) / (nR)

Where P is the poweer, MEP is the mean effective pressure, Vis the displacement volume and nR is the number of strokes per expansion. This makes it clear what we can do to get more power:

  • Increase either the mean effective pressure or displacement volume.
  • Decrease the number of strokes per cycle (such as with a two stroke engine).

Air to Fuel Ratios

Here is a list of the air to fuel stoichiometric ratios for different types of fuels. As a reminder, the air to fuel ratio is measured as the mass of air / mass of fuel.
  • Gasoline - 14.7
  • Diesel - 14.5
  • Ethanol - 9.0
  • Methanol - 6.4
  • LPG (Liquefied Petroleum Gas) - 15.5
  • CNG (Compressed Natural Gas) - 17.4
  • Hydrogen - 34

Volumetric Efficiency

The volumetric efficiency is the strictest constrain on power output and is the amount of air that an engine can breathe. If the engine has less air, less fuel will be delivered so less energy. 

Volumetric efficiency = 2ṁair / ρair*Vd*n

Ideal Spark Advance Combustion (not explosion)

One of the limiting factors of a ICE is the spark itself: how quickly can the fuel-air mixture ignite . For this reason, you will usually find that the spark will not ignite at top dead center (when the piston is compressing the mixture as much as possible) but a little bit before. This is so that when the piston reaches TDC, the whole mixture will have had time to ignite.
  • If the ignition happens after TDC, it is over advanced and will cause knocking.
  • If the ignition happens well before TDC, the cylinder pressure will reduce causing the performance of the engine to reduce.
The idea time for firing depends hugely on the engine and its characteristics. In essence, the below points will affect the ideal time for firing:
  • Engine speed
  • Cylinder temperature
  • Intake air temperature and humidity
  • Air to fuel ratio
  • Fuel type
  • Spark plug (props and numbers)
  • Camshaft timing
  • Geometric design of the combustion chamber

Energy Storage for Motorsport Vehicles

To help efficiency and improve performance, many powertrains are now storing energy and using it later on when it is required.There are mainly two types of energy storages used in motorsport:
  • Mechanical - This is typically a flywheel which stores the energy through rotating.
  • Electrical - This will either use a battery or a capacitor to store the energy.
There is also hydraulic storage but that is not really used as much as it is not as effective as mechanical and electrical storage systems for Motorsport purposes.

Battery vs Capacitor for Storing Energy

The battery works by moving electrons from one electrode to another. The flow of electrons creates a current which is used to power things. There are different types of batteries and electrodes used. Typically, the electrolyte would be lithium salts, the negative electrode be a carbon based material such as graphite and the positive electrode can range from lithium iron phosphate (LFP), lithium manganese cobalt oxide (NMC) or lithium cobalt oxide (LiCO).

The capacitor works by storing a positive charge on one plate that is insulated next to another plate with a negative charge. When connected to a circuit, the electrons move from the negative plate to the positive plate, causing a current which creates the power.
  • Charge rate - Capacitors can charge a lot quicker than batteries
  • Cooling demand - Batteries overheat much easier than capacitors.
  • Volume - To produce the same power, the capacitor will be a lot larger than the battery (battery is more energy dense).
  • Lifetime - A batteries lifetime will around between 100-500,000 cycles whereas a capacitor can last in its millions +.
  • Amount of energy that can be stored - Because the battery is more energy dense than the capacitor, it can store more energy.
  • Minimum operating temperature - A battery's minimum operating temperature will be around 5 degrees Celsius whereas a capacitor will be below -40 degrees Celsius.
  • Environmental impact - The battery will have a higher environmental impact than a capacitor.
For hybrid systems, you will tend to find that the battery would provide a large energy store. There is a capacitor too that would be able to accept large charges and slowly bleed it to the battery for storage, 
There are both problems with batteries and capacitors. The batteries need to become lighter and capacitors need to become smaller in size.

Electric Motors

There are a range of electric motors such as:
  • DC motors - These range from the brush DC motor, separately excited motor, series motor, permanent magnet DC (PMDC) and the brushless motor.
  • AC motors - These range from the induction motor to the synchronous motor.
  • Other motors - Other motors such as the stepper motor, hysteresis motor, reluctance motor and the universal motor.
For motorsport, the brush DC motor and permanent magnet DC (PMDC) are used.
  • Brush DC motor - This is used for drag racing as it provides a high starting torque and cheap. However, the brushes limit speed.
  • Permanent magnet DC - With a PMDC, there is no slip and precise control. It has a much lower current, good efficiency, good power density and a high continuous torque. However, it generates heat.
This makes it clear that cooling is the key challenge. Electrical circuits and loom range from temperatures between 150-700 degrees Celsius. The control unit and batteries should be kept under 65 degrees Celsius. 

Advanced ERSs in F1

The energy recovery system in F1 makes the F1 cars of today the first electricity powered cars in the history of motorsport. The motor generator can act as either the source of power or can supply electric power back to the battery, Both a proportion of the wasted kinetic and heat energy is recovered through ERS.

Now, we all know what the main question you are thinking, why were Mercedes so dominant in 2014?
  • The relative location of their of their turbo meant the engine's cooling was better.
  • The clutches used.
  • The ducting size.
  • The tight packaging of the whole engine, resulting to a lower centre of gravity.