Dynometer graph - how to read it?
Reading a graph from a dynometer is not difficult at all. The graph is arranged in such a way that on the horizontal axis the engine revolutions are presented, and on the vertical axis (or two vertical ones, usually on the left and right side of the graph) the values of power, torque and possibly other logged parameters (e.g. boost pressure, Lambda, air fuel ratio (AFR), exhaust temperature (EGT) or smoke index). To read a dyno graph, we need to distinguish how many measurements are on the graph. Typically, we have one or two measurements (in the latter case, these are usually: the measurement before modification or reference, and after modification - such as measurement after safe chip tuning). If one of the measurements is presented with a thinner line it is probably a reference measurement (this is what happens in dynometers that can't assemble more graphs into one). If there are more measurements, they definitely have different colors, making it easier to distinguish.
We will discuss two examples of a dyno graph - from Maha and ECU Tunes dynometer. Graphs from other dynometeres from other manufacturers can be read very similarly because they use the same data presentation scheme.
The car is a Porsche Cayman. We have superimposed two measurements on the graph. The reference measurement (stock version) has thinner lines. Wheel power (1) is shown in blue, resistance and loss power (2) in green, normalized engine power, the one we are most interested in (3) is drawn in red and finally normalized torque (4) in orange. On the horizontal axis the rpm, while on the vertical axis on the right side - both power and torque - so if, for example, the power is 400 hp and the torque is 400 Nm, the graphs of power and torque will reach exactly the same maximum level, although probably at different rpm.
The exact data of the maximum values are given in the table below, I have marked them with the corresponding numbers. Be aware - they are in a different order than on the chart legend (but have the right colors). The maximum power revolutions are under the loss power (but of course, these are the revolutions to the power drawn in red), the maximum torque revolutions (4) are under the torque. Below that, we still have the maximum speed and RPM of that speed.
On the right we have environmental data. (5) is the ambient temperature, (6) the temperature of the air drawn in by the car. Temperatures (5) and (6) should be close to each other. A difference of more than a few degrees means poor ventilation of the measurement hall or falsification of the graph (if you artificially inflate the intake air temperature, e.g. by giving the sensor under the hood - the DIN standard that normalizes the graph will greatly overestimate its result. It is also worth noting that there is no environmental data of the reference measurement in Maha's graph. It is just a background to the graph. Then we have (7) the atmospheric pressure. The dynamometers also measure humidity (above atmospheric pressure), but the effect of humidity on the result is small. The date and time of the reference measurement, as well as its environmental conditions, cannot be determined from the graph, but you can request a separate printout of such a graph and then these data on the graph will appear.
ECU TUNES DYNOMETER
The car is a Porsche Carrera. The graph is constructed similarly, we have two measurements on the graph. Since up to 4 measurements can be presented in ECU Tunes dynamometers, they have been color-coded differently. The color is shown in the table at the bottom, along with the measurement name. On the dynometer graph, we have normalized engine power (1), wheel power (2) and normalized torque (3). The corresponding revolutions at which this power and torque were obtained are listed next to the values. Ambient air temperature is (4), Atmospheric pressure is (5). At the top, we have the exact date and time the measurement was taken (6).
Air fuel ratio (AFR)
Thermal engines use fuel and oxygen (from air) to produce energy through combustion. To guarantee the combustion process, certain quantities of fuel and air need to be supplied in the combustion chamber. A complete combustion takes place when all the fuel is burned, in the exhaust gas there will be no quantities of unburnt fuel.
Air fuel ratio is defined as the ratio of air and fuel of a mixture prepared for combustion. For example, if we have a mixture of methane and air which has the air fuel ratio of 17.5, it means that in the mixture we have 17.5 kg of air and 1 kg of methane.
The ideal (theoretical) air fuel ratio, for a complete combustion, is called stoichiometric air fuel ratio. For a gasoline (petrol) engine, the stoichiometric air fuel ratio is around 14.7:1. This means that, in order to burn completely 1 kg of fuel, we need 14.7 kg of air. The combustion is possible even is the AFR is different than stoichiometric. For the combustion process to take place in a gasoline engine, the minimum AFR is around 6:1 and the maximum can go up to 20:1.
When the air fuel ratio is higher than the stoichiometric ratio, the air fuel mixture is called lean. When the air fuel ratio is lower than the stoichiometric ratio, the air fuel mixture is called rich. For example, for a gasoline engine, an AFR of 16.5:1 is lean and 13.7:1 is rich.
Exhaust Gas Temperature (EGT)
In a turbine engine, Exhaust Gas Temperature (EGT), sometimes referred to as Turbine Outlet Temperature (TOT), is the temperature of the turbine exhaust gases as they leave the turbine unit. The gas temperature is measured by a number of thermocouples mounted in the exhaust stream and is presented on a flight deck gauge in either degrees Fahrenheit or degrees Celcius.
In a piston engine, EGT is a measurement of the temperature of the exhaust gases at the exhaust manifold. As the temperature of the exhaust gas varies with the ratio of fuel to air entering the cylinders, it can be used as a basis for regulating the fuel/air mixture entering the engine.