A review of the characteristic parameters of reciprocating internal combustion engines will be used to support the presentation and discussion of the experimental results. The torque, power and overall performance, are three of the most important characteristic parameters in any internal combustion engine. The effective brake power (in kW) is given by equation (1).

$${\dot{W}}_{b}=B2\pi \frac{n}{60}{10}^{-3}$$(1)Where *B* is the torque and *n* is the rotational speed of the engine in revolutions per minute. The fuel consumption or mass flow rate of fuel is given by equation (2)

$${\dot{m}}_{f}=\frac{{m}_{f}}{\mathrm{\Delta}t}$$(2)Where: *ṁ*_{f} is the mass of fuel and *Δt* is the time interval. The overall efficiency is given by the ratio between the effective brake power and the thermal power supplied to the motor, expressed in equation (3). In turn, the thermal power is given by the product of the mass flow rate of fuel by the lower heating value of that same fuel.

$${\eta}_{g}=\frac{{\dot{W}}_{b}}{{\dot{m}}_{f}HV}$$(3)Where: *Ẇ*_{b} is the brake effective power, *ṁ*_{f} the fuel mass flow rate and *HV* is the lower heating value of the fuel. In the present case the fuel used is gasoline. For calculation purposes the value of 44000 kJ/kg for gasoline low heating value was considered [8].

In turn, the specific fuel consumption, *C*_{sf}, is given by equation (4). This parameter relates the fuel consumption with the effective brake power and allows to obtain a good term of comparison between engines.

$${C}_{sf}=\frac{{\dot{m}}_{f}}{{\dot{W}}_{b}}$$(4)In the technical literature, the specific fuel consumption is usually presented in g/kWh. Accordingly, equation 4 was reformulated as presented in equation (5).

$${C}_{sf}=\frac{{\dot{m}}_{f}h}{{\dot{W}}_{b}}$$(5)Where: *ṁ*_{fh} is the mass flow rate (g/h).

The consumption (per hour) of fuel, or mass flow rate of fuel in g/ h is given by equation (6).

$${\dot{m}}_{fh}=\frac{{m}_{f}}{\mathrm{\Delta}t}3600$$(6)The volumetric efficiency *η*_{V}, equation (7) [9], relates the amount of air actually introduced into the cylinder per cycle with the theoretical filling capacity of the cylinder in that same cycle. This is one of the most important parameters in the characterization and modeling of four-stroke internal combustion engines.

$${\eta}_{V}=\frac{{m}_{a}}{{m}_{at}}=\frac{{m}_{a}}{{\rho}_{ai}{V}_{d}}$$(7)Where: *m*_{a} is the mass that actually enters the cylinder in each cycle, *m*_{at}, is the mass that would theoretically fill the cylinder, *ρai*, the density of the air (or mixture) under atmospheric conditions and *V*_{d}, the displaced volume. In theory, the mass of the fresh charge in each cycle should be equal to the product of the density of the air (or mixture) evaluated at atmospheric conditions outside the engine by the displacement, *i.e*., the volume displaced by the piston. However, due to the reduced time available for the admission and load losses due to the existing flow restrictions, only a smaller quantity of the theoretical amount of fresh charge entering the cylinder under atmospheric conditions [10] eventually enters the cylinder. The value of the volumetric efficiency depends on several engine variables, such as engine speed, inlet and exhaust manifold pressures, and system geometry [11]. In this case, equation (8) is presented as the ratio between the flow rate actually admitted in the cylinder and the mass flow rate that would theoretically be admitted for that speed of rotation.

$${\eta}_{V}=\frac{{\eta}_{R}{\dot{m}}_{a}}{{\rho}_{ai}{V}_{d}\eta}$$(8)Where: *η*_{R} represents the number of revolutions per cycle and ṁ_{a} the mass flow rate that actually enters the cylinder. In practical terms, the value of the volumetric efficiency is obtained from the type of cycle, torque, fuel air ratio, air density, displaced volume, overall efficiency and lower heating value of the fuel as shown in equation (9), which results from the combination of equations (1) and (4), among others.

$${\eta}_{V}=\frac{{\eta}_{R}60{\dot{m}}_{f}AF}{{\rho}_{ai}{V}_{d}\eta}$$(9)Where: *AF* represents the fuel air ratio, considering the value of 14.7. The fuel air ratio (*AF*), equation (10), relates the mass of air to the mass of fuel *m*_{f} . These relationships can also be presented as the relation between mass flow rates.

$$AF=\frac{{m}_{a}}{{m}_{f}}=\frac{{\dot{m}}_{a}}{{\dot{m}}_{f}}$$(10)
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