EFFECT OF THROTTLING ON COMBUSTION IN A SMALL-DISPLACEMENT CARBURETOR ENGINE

A. S. TSEKHANOV

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

To clarify the nature of the change in the indicated efficiency of an engine, the study of the combustion process acquires great importance. It is therefore no accident that special attention is devoted to it. However, investigations [5, 8] and others on this question concern engines of large displacement and operating conditions with the throttle valve fully open. Below are presented some results of an experimental investigation of the effect of throttling on combustion in a small-displacement carburetor engine, carried out by the author of the present article in the laboratory of internal-combustion engines of the Tomsk Polytechnic Institute under the direction of Professor V. K. Nechaev.

The experiments were conducted on a two-cylinder L6/3 engine [1], a side-valve engine with a Ricardo-type combustion chamber, cylinder bore 65 mm, piston stroke 90 mm, compression ratio 5.26, operating on a balanced direct-current dynamometer.

Engine indication was carried out with a pneumatic-electric indicator with an electronic spark generator and a piezoelectric indicator, recording on a loop oscillograph. The investigation was conducted both with the most advantageous variable ignition advance and with a constant advance selected as optimal for full opening of the throttle valve.

The duration of the combustion phases was determined from indicator diagrams in degrees of crankshaft rotation. The heat-release coefficient was calculated according to E. K. Mazing [2]. All experimental results described below relate to engine operation according to the load characteristic at a constant speed of

\[ n = 1500 \text{ rpm}. \]

An investigation of the indicator diagrams of the throttled engine for mixtures from \(\alpha = 0.5\) to \(\alpha = 1.2\) shows that, when the most advantageous variable ignition advance is set, the beginning of the period of visible combustion (at the moment when the combustion line separates from the compression line on the indicator diagram, Fig. 1a) occurs at \(4\text{–}7^\circ\) before TDC; the end of combustion, by the pressure maximum [11], occurs at \(15\text{–}19^\circ\) after TDC. The point of maximum cycle pressure is taken as the end of combustion, since by this moment the flame front passes through the entire combustion chamber, reaching its outermost points [5, 9, 10]. Thus,

Fig. 1. Effect of throttling on the combustion phases, \(\alpha = 1.17\):
a) most advantageous variable ignition advance; b) constant ignition advance.

the duration of the period of visible combustion $\varphi_{\text{vis}}$ under throttling remains constant (Fig. 2) almost up to the very limits of inflammability of the above-mentioned mixture compositions. The independence of the duration of the period $\varphi_{\text{vis}}$ from the physicochemical properties of the mixture (mixture composition and residual gases) can be explained by the formation of a turbulent flame front [3, 4], in which, owing to the strong fragmentation of the flame surface by turbulent pulsations, the flame speed is determined by the mass burning rate of the mixture throughout its entire volume.

The constancy of the duration of the period $\varphi_{\text{vis}}$ gives us grounds to consider that an increase in the fraction of residual gases in the working mixture has no effect on the rate of turbulent combustion. In the opinion of A. S. Sokolik [6], residual gases during throttling of the engine do not affect the rate of turbulent combustion, but affect the rate of chemical combustion, i.e., heat release. However, when the L6/3 engine was throttled down to $\eta_v = 0.4$, despite the undoubted increase in the fraction of residual gases in the mixture, no deterioration in the completeness of combustion was observed. On the contrary, the heat-release coefficient $\xi_z$ increased somewhat (Fig. 3). This is evidently facilitated, along with the turbulent factor, by the high temperature of the residual gases.

The values of $\xi_z$ presented in Fig. 3 for rich mixtures include incomplete combustion due to a lack of oxygen.

Residual gases, when their content in the mixture is increased, heat the latter more rapidly throughout its entire volume, thereby ensuring, under intensive turbulent motion of the burning gases, good combustion of both rich and lean mixtures; this explains the stable operation of the engine under throttling. The latter is also confirmed by the absence in our experiments of incomplete and misfired cycles on the oscillograms of continuous recording of pressures in the engine cylinder (Fig. 4), taken under the regimes noted above.

Fig. 2. Dependence of the duration of the combustion phases on the volumetric efficiency $\eta_v$ under throttling; ○—most advantageous variable ignition advance; △—constant ignition advance.

From Fig. 4 it is evident that the scatter of flash pressures $p_z$ from cycle to cycle is almost the same, at equal volumetric efficiencies, both for a rich mixture $\alpha = 0.8$–$0.9$ and for a lean mixture $\alpha = 1.2$.

As the study of the indicator diagrams showed (Fig. 5, Table 1), the combustion temperature reaches its maximum value (polytropic exponent $n_2 = 1.0$) only after the maximum combustion pressure $p_z$ has been reached; this can be explained by intensive afterburning of the mixture after the flame has passed through the combustion chamber in small foci formed as a result of fragmentation of the flame front by turbulent pulsations [3, 4, 5]. The farther to the right the ...

Fig. 3. Dependence of the heat-release coefficient \(\xi_z\) on the filling coefficient \(\eta_v\) under throttling; the ignition advance is the most favorable variable.

\[ \eta_v = 0.345 \]

\[ \alpha = 0.83 \qquad \alpha = 1.20 \]

\[ \eta_v = 0.292 \]

\[ \alpha = 0.87 \qquad \alpha = 1.22 \]

\[ \eta_v = 0.231 \]

\[ \alpha = 0.85 \qquad \alpha = 1.18 \]

Fig. 4. Oscillograms of gas pressure in the cylinder of the Л6/3 engine under throttling; the ignition advance is the most favorable variable.

on the indicator diagram, the point corresponding to the maximum combustion temperature ($n_2=1.0$) relative to the maximum combustion pressure $p_z$, the greater, obviously, is the afterburning.

Fig. 5. Change in the polytropic exponent of expansion over the piston travel under throttling, ignition advance set to the most favorable value, $\alpha=1.07$.

Table 1

Position of the points $n_2=1.0$ (maximum combustion temperature) on the combustion–expansion line, in degrees from TDC under throttling, as a function of mixture composition

With the closing of the throttle valve, as is seen from the graph and the table, the position of the points $n_2=1.0$ on the expansion line does not change. This indicates that an increase in the fraction of residual gases in the mixture does not affect the quality of combustion. The position of the point $n_2=1.0$ (Table 1) is affected by the mixture composition. The smallest displacement of the point $n_2=1.0$ from TDC toward the expansion process is observed for mixtures $\alpha=0.8—0.9$, burning with the maximum normal velocities. With enrichment and leaning of the mixture from the indicated composition, the duration of afterburning increases and the point $n_2=1.0$ shifts from TDC toward expansion. This indicates that in the zones

afterburning (the third phase of combustion according to A. S. Sokolik [3]) the combustion rate is determined only by the physicochemical properties of the charge, since the foci are small in comparison with the predominant scales of the acting turbulent pulsations [3, 4]. Photographs of the flame in the engine cylinder obtained by a number of authors [5, 9, 10] also clearly show luminosity after the flame has passed through the entire combustion chamber, which is explained by afterburning of the mixture in foci that have remained as a result of fragmentation of the flame front. The improvement of combustion with an increase in the fraction of residual gases, similar to that considered, is also noted in works [7, 12] and is explained by internal heating of the mixture by the residual gases.

With further throttling of the engine, the coefficient $\xi_z$ decreases somewhat. Apparently, the increase in residual gases in the mixture at $\eta_v < 0.4$ reduces the completeness of combustion because incomplete combustion products are formed owing to the low oxygen concentration and low pressures in the cylinder.

The influence of residual gases is manifested chiefly in an increase in the duration of the ignition-delay period $\varphi_{zb}$ (Fig. 2) and leads to a lengthening of the duration of complete combustion $\varphi_{\text{pol}}$. The combustion rate during the period $\varphi_{zb}$ is determined by the physicochemical properties of the charge—the composition of the mixture and the residual gases [3, 4, 5]. The combustion focus during this period is very small in comparison with the scale of turbulent pulsations, and the latter do not influence the acceleration of combustion. An increase in the fraction of residual gases in the mixture during throttling reduces the normal combustion rate, and the duration of the period $\varphi_{zb}$ increases.

The shortest duration of the period $\varphi_{zb}$ is attained for mixtures of composition $\alpha = 0.8$–$0.9$ (Fig. 2), which burn with the highest normal rates. With enrichment and leaning of the mixture from the indicated composition, the normal combustion rate decreases and the duration of the period $\varphi_{zb}$ increases. With the setting of the most advantageous variable ignition advance, the period $\varphi_{zb}$ shifts toward the compression process, thereby stabilizing the position of the period $\varphi_{\text{vid}}$ along the piston stroke.

When the engine operates with a constant ignition advance (equal to $14^\circ$ before TDC), selected as optimal for the full-throttle operating condition, under throttling the duration of the period $\varphi_{zb}$ (Fig. 2) does not differ from its value with the most advantageous variable ignition advance. Lengthening of the period $\varphi_{zb}$ as the throttle is closed leads to a shift of the period $\varphi_{\text{vid}}$ toward the expansion stroke (Fig. 1b). The degree of expansion is thereby reduced, as a result of which the amount of heat converted into indicated work decreases.

For mixtures $\alpha = 0.7$–$1.0$, the duration of the period $\varphi_{zb}$ is minimal (Fig. 2) and does not cause a large displacement of the period $\varphi_{\text{vid}}$ from TDC. Here combustion proceeds under the same conditions as with the most advantageous variable ignition advance, and the duration of the period $\varphi_{\text{vid}}$, like $\varphi_{\text{pol}}$, does not differ from its value with the most advantageous ignition. When the mixture is leaned from $\alpha = 1.0$ to $\alpha = 1.2$, with a decrease in the volumetric efficiency coefficient $\eta_v$, the duration of the period $\varphi_{zb}$ (Fig. 2) increases and causes a shift of visible combustion from TDC to the right.

When throttling to $\eta_v = 0.4$, the duration $\varphi_{\text{vid}}$ also practically does not differ from its value with the most advantageous variable ignition advance, despite the shift of visible combustion into the expansion stroke to $35^\circ$ after TDC. The latter

indicates the presence of intense turbulence in a small-displacement engine even when the piston is considerably removed from TDC. The increase in the duration of the period \(\varphi_{\text{vis}}\) at partly closed throttles indicates less intense turbulence [5] when the piston is farther from TDC. The total duration of combustion under throttling with constant ignition advance increases, in comparison with the most favorable ignition advance, by 7–15%.

The experimental investigations described above make it possible to draw the following conclusions:

1. When variable ignition advance is set, regardless of the mixture composition within the flammability limits, the duration of visible combustion remains constant at any positions of the throttle valve. The beginning of visible combustion occurs at \(4–7^\circ\) before TDC, and the end at \(15–19^\circ\) after TDC.

The duration of the ignition-delay period depends on the physicochemical properties of the mixture. Enrichment and leaning of the mixture from the composition \(a = 0.8–0.9\), as well as an increase in the content of residual gases in it when the engine is throttled, lead to a lengthening of the ignition-delay period and to an increase in the duration of complete combustion.

The heat-release coefficient practically does not change when the throttle valve is partly closed. This indicates that, with an increase in the degree of throttling of the engine, combustion does not deteriorate. The latter is also confirmed by the unchanged position, along the piston stroke, of the points of maximum combustion temperature \(T_z\) (points \(\bar n_2 = 1.0\)) at different throttle-valve openings.

1. At a constant ignition-advance angle, selected as optimal for full throttle, the duration of the ignition-delay period does not differ from its value at the most favorable variable ignition advance. An increase in the duration of the ignition-delay period shifts visible combustion onto the expansion stroke. When the charge is reduced to \(\eta_v = 0.4\), the duration of visible combustion practically does not change; with further throttling, the duration of visible combustion increases.

REFERENCES

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