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Engine tuning
2017 Mattia Piron. All rights reserved.

  1. Introduction
  2. Piston surface
  3. Air density
  4. Efficiency
    1. Sizing the intake funnel
    2. Sizing the intake duct
    3. Airbox
    4. Exhaust system
  5. Average piston speed
  6. Bibliography

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Immagine di copertina

SEPTEMBER 20th, 2018


Introduction       top

To increase the power of 4-stroke engines, we can work on several areas. Power is given by:


  • η = useful engine efficiency;
  • ρ 0 = air density [kg/m3];
  • Z = number of cylinder;
  • S = piston surface [m2];
  • n = number of cylinder;
  • Hi = fuel calorific value [J/kg];
  • u = average piston speed [m/s];
  • α t = air-to-fuel ratio;
  • T = strokes of engine [2 or 4].

In compact form, we can write:

Where PME is Mean Effective Pressure. Using the proposed units of measurement, the power thus obtained will be in watts. It will be sufficient to divide that value by 735 to obtain the power in horsepower. If we want to increase the horsepower of an engine, we will have to increase the numerator values in the formula, or decrease the denominator values. We cannot increase the number of cylinders in an existing engine, the air/fuel ratio for an Otto cycle engine will have to be about constant (although this formula shows that a slightly lower value, i.e. a rich mixture, leads to more power), the calorific value of the fuel does not vary, unless we use special blends or fuels (an example is speedway bikes, which burn methyl alcohol), and the engine timing is fixed. We can work on:

  • Piston surface;
  • Air density;
  • Efficiency;
  • Average piston speed.


Piston surface       top

There is not much to say about this. Replacing cylinders and pistons with larger ones will produce an immediate increase in power. This is probably the most common form of tuning, especially in 2-stroke single cylinders under 125cc. Needless to say, the increased weight of the piston, and the consequent increase in the force of the gases pushing on the surface of the piston, leads to a decrease in the life of the connecting rod (and/or crankshaft and main bearings).


Air density       top

The denser the air is, the more gasoline is mixed with it, and consequently the more energy the combustion chamber will have. This is intuitive, and noticeable to anyone who, going up in the mountains, will notice a certain loss of power in their engine. This represented a real problem when, in the past, internal combustion engines equipped aircraft that found themselves at altitude with the power halved compared to the start. Thus the turbocharger was born. It allows a greater quantity of air to enter the combustion chamber, moreover the inlet pressure is controlled and kept constant, even with changes in atmospheric pressure. For this reason those of you who drive a turbocharged car will probably not notice any loss of power going up the mountain.


Efficiency       top

Every fuel contains a certain amount of energy. By burning that fuel inside a combustion chamber, we convert this energy into power. Only 15-20% of this energy actually becomes mechanical power transmitted to the wheels. The rest is lost in friction and heat. Basically, a tuner's job is to increase this efficiency as much as possible while reducing losses. Mechanical losses must be reduced, i.e. all those unwanted slippages due to tolerances that are too wide or misalignments of shafts and gears, and aerodynamic losses, i.e. ducts with curves that are too sharp, or with bottlenecks, or with too rough surfaces that lead to unwanted turbulence in the fluid. The compression ratio is increased, increasing the risk of detonation, or self-ignition of the air/fuel mixture. The entire surface of the combustion chamber must be polished to a high gloss, and any edges must be removed, in order to reduce the presence of ignition points. Exhaust ducts should also be polished to a high gloss, reducing the risk of carbon deposits adhering. Intake ducts should not be polished to a mirror finish, as this would have the effect of causing the air to "stick" to the walls. They must maintain a certain surface roughness to allow the presence of an aerodynamic boundary layer, improving airflow.


Sizing the intake funnel       top

The intake funnel is the final portion of the intake duct, having the shape of a trumpet (or a cone). Its presence is necessary to help the entry of air into the intake duct, and provide a sort of "supercharging effect". The maximum area of the intake trumpet will be:

  • At = maximum funnel area [m2];
  • u = average piston speed [m/s];
  • S = piston surface [m2];
  • Cs0 = sound speed [m/s];

At the end of the maximum area of this funnel, there shall be a so-called "mouthpiece", which is a rounded area that guides the fluid from the external environment to the beginning of the funnel. This zone will ideally have an area twice as large as the maximum area of the nozzle.


Sizing the intake duct       top

The suction line starts at the intake valve(s) and ends at the funnel inlet. It will need to have a certain length, defined by the following formula:

  • L = length from the intake valve to the end of the funnel [m];
  • Cs = sound speed, you can take 340 [m/s];
  • n = maximum power regime [RPM];
  • K = Cylindrical fraction of the duct. If K = 0.5, half of the duct will be cylindrical, the other half will be conical;
  • u = average piston speed [m/s];
  • S = surface of one piston [m2];
  • Ac = cylindrical zone area [m2];
  • Amt = average area funnel zone [m2]


Airbox       top

The sole purpose of the intake box (filter box) is to constantly provide fresh air to the duct. In fact, in a well-sized duct, this would not be necessary. However, there is a risk of the engine sucking in hot air (if the intake trumpets are inside the engine compartment) or turbulent air. The presence of the filter box allows for an area containing fresh air that is always present, and maybe even at a higher pressure than atmospheric. In the case in which the filter box has to work also as an intake silencer, and consequently the duct has to be of small section, it has to be sized exploiting the helmholtz resonance, through the following formula:

  • A = duct area [m2];
  • V = airbox volume [m3];
  • L = duct length [m];
  • n = maximum power regime [RPM];
  • Z = number of cylinder;
  • Cs0 = sound speed, you can take 340 [m/s];
  • T = strokes (2 or 4) of the engine

An infinite number of combinations are possible. You can impose a given volume and length of the duct, and through this formula you calculate the area, or you impose the area and volume and calculate the length, and so on.


Exhaust system       top

In a multi-cylinder engine, the exhaust system consists of pipes connected directly to the engine header (the exhaust manifolds), which join into a single end pipe. In reality many configurations are possible, we limit ourselves in this website to give only a general overview. The diameter of the exhaust manifolds is chosen empirically, and generally has a section equal to 1.4-1.5 times the minimum section of the intake pipe. The length is calculated using the following formula:

  • L = manifold length [m];
  • Ts = average exhaust gas temperature [K];
  • Ta = intake air temperature [K];
  • n = maximum power regime [RPM];
  • θ = aperture timing angle exhaust valve [rad];
  • u = average piston speed [m/s];
  • S = surface of one piston [m2];
  • As = surface of exhaus pipe [m2];

The gas temperature to be used in the formula is the average temperature. To calculate it, it can be considered that the temperature of the gases leaving the cylinder decreases by about 100 C every 20 diameters of length. The manifolds will flow into an exhaust terminal, having length:

  • Lt = exhaust terminal length [m];
  • Zt = number of cylinders;
  • Vu = unit displacement [m3];
  • St = exhaust terminal surface [m2];
  • ηv = engine volumetric efficiency;

The length calculated in this way may be too long, in which case you can use a submultiple of it.


Average piston speed       top

The pistons of purely sports engines exceed 20 m/s average speed, and can reach 25 m/s. Quieter engines have much lower values, around 15-18 m/s, while diesel engines may not even reach 10 m/s. Increasing this value is not trivial, as the intake and exhaust ducts must also be adequately sized. The biggest problem, however, is in the mechanical strength. Such speeds imply significant accelerations of the connecting rod/piston assembly, resulting in very high dynamic forces. Each component must be designed to withstand these stresses, so it is not possible to increase this value too much.


Bibliography       top

  1. Motori ad alta potenza specifica, Giacomo Augusto Pignone, Ugo Romolo Vercelli, Giorgio Nada Editore, 1995
  2. Elaboriamo il motore - Testate, F.L. Facchinelli, Motor Books Tech, 2003
  3. Elaboriamo il motore - Manovellismo e distribuzione, F.L. Facchinelli, Motor Books Tech, 2005


Help me help you         top

You should know that I created this website out of passion and love for mechanics. If you would like to help me to maintain these pages, you can make a small donation as well. Thank you so much.

Please help Mattia Piron

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