Effect of Load on the Efficiency of the Theoretical Open Cycle of a Carburetor Engine

A. S. Tsekhanov

(Presented by Prof. V. K. Nechaev, Doctor of Technical Sciences)

In the author’s previous work [3], a study of an ideal cycle was carried out. In that analysis, however, no account was taken of the change in the composition of the working fluid during the course of the cycle. Somewhat closer to the actual working process is the so-called theoretical open cycle (Fig. 1), proposed by I. M. Lenin [2]. Below are presented the results of a study of the efficiency of this cycle as a function of load.

Fig. 1. The theoretical open cycle of a carburetor engine.

In the theoretical open cycle [2], owing to the absence of hydraulic and, consequently, pumping and mechanical losses, the mean cycle pressure (mean indicated pressure) $p_i$ is an indicator of load. Quantitative regulation is carried out by chan-

by varying the pressure at the beginning of compression and is evaluated by the filling coefficient $\eta_v$. As the working medium we take a gasoline–air mixture, whose composition is characterized by the excess-air coefficient $\alpha$, and its combustion products; the residual gases are the real combustion products.

In the calculations the following initial data were adopted:
fuel—automobile gasoline of average composition: $C=0.855$; $H=0.145$.
The lower heating value of the fuel is $H_u=10400\ \text{kcal}/\text{kg}$.
The pressure and temperature of the environment are $p_0=1\ \text{kg}/\text{cm}^2$, $T_0=288^\circ$ abs.
The pressure and temperature of the residual gases are $p_r=1\ \text{kg}/\text{cm}^2$, $T_r=1000^\circ$ abs.
The compression ratio is $\varepsilon=5.26$.

The cycle calculation was carried out by the method of Grinevetskii–Mazing [1]. The parameters at the end of filling were determined according to I. M. Lenin [2], taking into account the expansion of the residual gases down to the intake pressure.

The results of calculating the theoretical open cycle (Fig. 1) are summarized in the table.

In this table the following notation is used:
$T_{yr}$—temperature of the residual gases expanded to the intake pressure;
$\kappa_r$—adiabatic exponent for the expansion of the residual gases to the intake pressure;
$\kappa_1$, $\kappa_2$—adiabatic exponents for compression and expansion;
$\mu$—coefficient of molecular change;
$\gamma$—coefficient of residual gases;
$\lambda$—degree of pressure rise.

The dependence of the indicated efficiency $\eta_i^T$ of the open theoretical cycle on load, according to the calculation data, is also shown in Fig. 2. From this graph and the table it follows that a decrease in the theoretical mean indicated pressure $p_i^T$ under quantitative regulation leads to a certain increase in the theoretical indicated efficiency $\eta_i^T$.

Fig. 2. Influence of the mean indicated pressure of the theoretical cycle on the indicated efficiency of a throttled carburetor engine.

Thus, for the mixture compositions considered, when $p_i^T$ decreases from 12 to $2\ \text{kg}/\text{cm}^2$, $\eta_i^T$ increases by 1.5–2.5%. With enrichment of the mixture from the normal composition $\alpha=1.0$, the coefficient $\eta_i^T$ falls sharply because of theoretical incomplete combustion, but the regularities of the change in $\eta_i^T$ for constant mixture compositions, both rich and lean, are the same with changing load.

As the quantity of the gasoline–air mixture decreases, the relative content of residual gases $\gamma$ increases, which causes a decrease...

Table 8

Calculated data of the theoretical open cycle, \(\varepsilon = 5.26\)

the heat input per unit of the entire working medium in the cylinder (fresh mixture plus residual gases), despite the constancy of the composition of the fresh mixture, and leads to a decrease in the end-of-combustion temperature $T_z$ and in the pressure-rise ratio $\lambda$ (Fig. 3). The decrease in the temperature $T_z$ with a decrease in $p_i^T$ leads to a reduction in the heat capacity of the gases and, correspondingly, to an increase in the expansion adiabatic exponent $k_2$. The compression adiabatic exponent $k_1$, on the contrary, decreases as $p_i^T$ is reduced. This is caused by an increase in the heat capacity of the working medium along the compression line, owing to the growth in the fraction of residual gases as $p_i^T$ decreases. But the change in the heat capacity of the working medium during the expansion process has a greater influence on the efficiency of the cycle than the heat capacity during the compression process. Its decrease leads to a reduction in the heat carried away by the gases from the cylinder and to an increase in the efficiency of the theoretical cycle.

The study carried out of the theoretical open cycle of a carburetor engine under quantitative regulation (corresponding throttling) makes it possible to draw the following conclusions.

Fig. 3. Dependence of the end-of-combustion temperature $T_z$, the pressure-rise ratio $\lambda$, and the adiabatic exponents of compression $k_1$ and expansion $k_2$ on the theoretical mean indicated pressure $p_i^T$.

1. With decreasing load, the theoretical indicated efficiency increases.

2. The end-of-combustion temperature and the pressure-rise ratio decrease with decreasing load, despite the constancy of the composition of the fresh mixture, as a result of the increase in the fraction of residual gases.

3. With decreasing load, the compression adiabatic exponent decreases, while the expansion adiabatic exponent increases.

REFERENCES

1. Mazing E. K. The Thermal Process of Internal-Combustion Engines, ONTI, 1937.

2. Lenin I. M. Working Processes and Carburetion in Automobile Engines, Mashgiz, 1947.

3. Tsekhanov A. S. Influence of load on the thermal efficiency of a carburetor engine (the article is included in this volume).