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Undoubtedly, their designers can be both technically literate people and those who are not sufficiently familiar with the basic provisions of the ICE theory. In this article, we will try to give an analysis of the engines presented at the last Moscow rally of ultralight aircraft, and a few tips on choosing the parameters of the internal combustion engine, the observance of which will shorten the relatively expensive and long search path, and will help to significantly reduce the likelihood of technical risk.

All internal combustion engines of aircraft presented at the rally can be divided into three categories:

1. Serial (boat, motorcycle, ICE from snowmobiles, automobiles), adapted without major alterations.

2. Own design, with a wide use of parts of serial motors.

3. Original developments, made from scratch.

These motors, including competitive ones, are summarized in Table No. 1. Column 1 shows their effective maximum power N_emax, spent on the rotation of the propeller, with the help of which the torque on its shaft M_cr converted to axial thrust. To judge the power of the power unit, build the characteristics of the propeller group, select the propeller and link it with the engine, you need to have an external characteristic, a curve of maximum power that the engine can develop at different speeds with a fully open throttle. Accurate data can be obtained by testing it on brake stands, which is not available to every amateur. There is an approximate way to build an external characteristic based on theoretical calculations, if there is at least one point of power and crankshaft speed (they are usually indicated in the factory data).

Table 1 (click to enlarge)

(click to enlarge)

This method consists in the fact that, at a constant composition of the fuel mixture, the power expended to overcome internal losses varies approximately in proportion to the square of the number of revolutions.

N₁ - indicator power, l. With.;

N_mp - the power expended to overcome the friction forces of the pistons, pumping losses during purge, rotation of ignition units, distribution, etc.;

N_e - effective power;

N₁', N_mp', n' rpm - current power and rpm values.

N₁'=N₁*(n'/n), (1)

N_mp'=N_mp*(n'/n)2. (2)

For example, we will construct the external characteristics of the RMZ-640 engine in this way.

Factory declared maximum effective power:

N_emax= 27 l. With. at 5250 rpm.

We accept mechanical efficiency η_м=0,87, then the indicator power N₁=N_emax/η_м\u27d 0,87 / 31 \uXNUMXd XNUMX l. With.

Friction power: N_mp=N₁-N_emax\u31d 27-4 \uXNUMXd XNUMX l. With.

Let us determine by formulas (1, 2) N₁', N_mp', N_e', pre-set by a number of values ​​of revolutions n rpm, and summarize the results in table. 2. Based on these data, we build the external characteristic N_e=f(n) (Fig. 1).

Table 2

Rice. 1. External characteristics of the RMZ-640 engine

There are maximum (or take-off), rated and operational maximum power. Maximum power N_emax obtained when the engine is running at full throttle on the ground. This mode for the engine is stressful and is limited to 3-10 minutes. Power less than the maximum by 10-15% is called nominal (N_{e nom}). You can use it for a long, but limited time, no more than 1-1,5 hours. Operating power (N_{e ex}) is less than the maximum by 25-30%, the engine operation time at this power is not limited.

The turns corresponding to the types of capacities are called maximum, nominal and operational. By itself, engine power does not yet indicate its merits, since it must be correlated with its mass (see column 2).

The flight mass of the propeller unit includes the mass of all units necessary for flight, with tanks filled with oil and fuel.

The flight weight as an objective criterion of the weight quality of the engine is inconvenient because it takes into account consumable goods (fuel, oil), depending on the purpose and type of aircraft. The total mass of these components is not easily defined, so the motor mass is characterized by a less complete, but more precisely defined concept of dry mass.

Column 3 shows a comparative assessment of motors of different power in terms of specific gravity.

g=G_dv/N_emax,

where G_dv - dry weight of the engine, kg; N_emax - maximum power, l. With.

When calculating the specific gravity, as a rule, the dry mass of the motor is referred to the maximum power. Specific gravity is one of the most important indicators of the quality of an aircraft engine.

The specific gravity of modern Western internal combustion engines for ALS is 0,5-0,6 kg/l. s., in the best representatives of 0,25-0,4 kg / l. With. For example, the specific gravity of two-stroke internal combustion engines for ALS of the American company "Kolbo Corp":

g kg/l. With.             N_emax l. from.

0,32 6

0,25 18

0,23 25

The competition engine M-18 has the best indicator: g=0,34 kg/l. s., the worst 2,04 kg / l. With. at the engine "Dnepr" MT-10.

It is known from the theory of similarity that for geometrically similar engines, the mass is proportional to the cube of the cylinder diameter, and the power is proportional to the square of the diameter, that is

g=G_dv/N_emax=A*(D³/D²)=AD.

In practice, this relationship is not observed, because a strict geometric similarity between parts of the same name of various sizes is impossible because the sections of many parts are specified by production conditions; casting thickness, rigidity, installation conditions, etc., therefore, these cross-sectional dimensions can be considered constant. Then: G_dv=AD². Statistics show that medium and large size engines follow this relationship well, thus:

g=G_dv/N_emax=A*(D²/D²)=A=const.

This dependence is violated in the region of small D in the direction of increasing mass and is explained not only by the above technological reasons, but also by the fact that the mass of service units - magnetos, candles, carburetors, etc. - little depends on the size of the motor. The relative mass of these parts, which is insignificant for large engine sizes, increases with a decrease in engine volume (Fig. 2).

Rice. 2. Dependence of the engine specific gravity on the displacement

Column 4 shows the values ​​\uXNUMXb\uXNUMXbof liter power, this value is an important parameter for the perfection of the motor.

As you know, the power of the motor:

N_emax=(P_e*V_s*n_Max)/(225*i), where

P_e - average effective pressure, kg / cm²,

V_s - engine displacement, cm³,

n - rotation speed, rpm,

i - tact.

From here, the liter power will be expressed:

N_л=N_emax/V_л, l. s./l.

With an increase in liter power, the dimensions of the engine and its weight are reduced. In terms of liter power, the two-stroke engine IZH-Sport, N has the highest performance._л= 91,5 l. s. / l, the smallest for a two-stroke Skoda engine is 39 liters. s./l. About 80% of the presented engines have N_л from 46 to 63 liters. s./l.

The widely used in the West two-stroke engines for ALS "Rotaps", "Hirt", "Kyun", "Kawasaki" - Nl = 80 ... 105 liters. s./l. Thus, the engines presented at the rally have reserves for forcing.

From the theory of similarity, it is known that the liter capacity is inversely proportional to the diameter of the cylinder, that is:

N_л=A/D, while

f_cool=F_cool/U_s=D²/D³=A/D

where f_cool is the ratio of the cooling surface to the volume of the cylinder,

F_cool - cooling surface,

U_s is the volume of the cylinder,

that is, as the cylinder diameter decreases, the cooling surface area per unit volume increases, which improves the cooling of a small diameter cylinder, increases heat loss and reduces thermal efficiency η_t, but at the same time this makes it possible to increase the compression ratio and compensate for the drop in η_t, that is, an increase in thermal efficiency should not be expected.

Let's try to decide which engine is more suitable for the SLA - four-stroke or two-stroke. Let's start with fuel consumption. A two-stroke internal combustion engine has 400-450 g/hp, a four-stroke internal combustion engine has 200-250 g/hp, that is, the specific consumption of a two-stroke engine is on average 2 times higher than that of a four-stroke one. But the latter may turn out to be less beneficial for the ALS because of the greater mass and greater air resistance, since part of the effective power will be spent on moving the heavier engine in the air and overcoming its harmful resistance. Therefore, the efficiency of the flight is most fully characterized by fuel consumption per ton-kilometer.

This indicator, in addition to efficiency, also takes into account the amount of air resistance of the propeller installation, the efficiency of the propeller and a number of other indicators, in a word, the whole set of factors that determine the degree of perfection of the aircraft.

The use of new materials, technologies, profiles for the manufacture of ALS will lead to the appearance of a design with a low weight of the airframe. In this case, the relative mass of HMG will increase even more. Four-stroke engines will have an undeniable advantage over two-stroke engines on long-haul flights, when specific fuel consumption becomes decisive.

We have already talked about the effect of cylinder volume (see Table 1) on specific gravity and liter power. Now consider the effect of cylinder size on the indicator efficiency. Recall that the indicator efficiency η_і - the ratio of thermal energy converted into work to all supplied to the engine.

Thus, the thermal efficiency of small-sized internal combustion engines will be relatively low, and their specific fuel consumption will be higher.

Table 1 gives the dimensions of the cylinder, piston and its relative stroke S/D. These parameters are closely related, so let's consider them together.

The next indicator is the piston speed

V_Wed=(S*n)/30, where

S - piston stroke, m; n - crankshaft speed, rpm. The average piston speed for the engines presented in the table is from 8,4 m/s to 17 m/s. This indicator seriously affects the dynamic load of engine parts, cylinder filling and the amount of energy expended on the friction of pistons and bearings. The average piston speed of special engines for ALS is 12-15 m/s.

The crankshaft speed (see table 1) of the considered power plants is from 4500 rpm to 8000 rpm. It is known that the power of an internal combustion engine depends on its speed. However, forcing is accompanied by a sharp (proportional to the square of the number of revolutions) increase in the inertia forces of the rotating and translationally moving masses of engine parts and, as a result, an increase in friction losses, which requires strengthening the mechanical strength of engine parts and changing the operating conditions of the bearings. On the other hand, the increase in speed is limited by the cooling of the cylinder head, piston, candles, since with an increase in speed, heat removal from the cylinder increases. In addition, the speed of rotation is limited by the average speed of the piston, with an increase in which the hydraulic losses on the purge increase sharply (in proportion to the square of the piston speed), which reduces filling and reduces engine power. At the same time, increasing the rotation frequency to a certain limit improves η_і.

Table 1 also shows the average effective pressure and compression ratio. It can be seen from the power formula that there are two main directions for increasing power - this is an increase in speed and pressure P_e. We discussed the effect of RPM on power earlier. Let's see how we can raise R_e.

This is easily achieved by increasing E - the compression ratio (for two-stroke engines, the effective compression ratio is used).

E_eff=(V_eff+V_kc) / V_kcWhere

E_eff is the effective volume described by the piston from the upper edge of the exhaust port to TDC, V_kc - volume of the combustion chamber (see Table 3).

Table 3

Graph of the effect of increasing compression ratio (solid lines) and boost (dashed lines) on pressure at the end of combustion. P_z and specific fuel consumption C_e (at %)

This method is good because it is simple and, in addition to increasing power, leads to a decrease in fuel consumption. However, it also has disadvantages.

An increase in E is accompanied by an increase in temperature and pressure at the end of the compression stroke, causing a sharp increase in combustion pressure P_e, and consequently, causes the need for more durable parts, tightens the requirements for fuel and oil. However, the effect of increasing power from increasing P_e has physical limits - more than 15-20%, so the power cannot be increased. With compression ratios of 10-12, the increase in power is already insignificant. To what extent can the compression ratio be increased from the point of view of practical benefits? Rise P_z and η_t can be traced as E increases from 4 to 8. Omitting the calculated side, we present the result.

Compression ratios E equal to 4, 5, 6, 7, 8 correspond to combustion pressures P_z 25,3 kg / cm², 34 kg/cm², 44,0 kg/cm², 54,2 kg/cm² and 65,5 kg/cm². This shows that as E increases from 7 to 8, we gain in efficiency η_t only 4,6%, while the combustion pressure rises from 54,2 to 65,5 kg/cm20, i.e. by XNUMX%. Therefore, in practice, a compromise must be made between the optimal compression ratio and η_t (see graph).

For practical use, it is possible to recommend the values ​​of the most favorable compression ratios when operating on fuel that does not detonate under all circumstances.

Another way to increase R_e is to increase the pressure of the mixture at the inlet.

For two-stroke engines, an increase in P_e is achieved by using resonant pipes in the intake and exhaust (the Cadenasi effect, discovered by him in 1903 and first implemented on the Yumo engine in 1923, when a 60% increase in power was obtained). A tuned exhaust system, for example, increases power by up to 30-40% without a large increase in engine mass, while also improving its efficiency.

Raise P_e four-stroke engines are much more difficult. Even a simple change in the valve timing will put the designer in front of a serious technological and design task of manufacturing a camshaft, boring seats and installing new valves, etc.

Our statistics give the following P_e: for four-stroke internal combustion engines from 9,5 to 10 kg/cm², two-stroke have from 3,6 to 6,6 kg / cm², for 40% of two-stroke engines Р_e ranges from 5,1 to 6,5 kg/cm², which is a good indicator. At the same time, the RMZ-640 engine (one of the most common at the rally) has R_e is only 3,6 kg/cm², which indicates the reserves for increasing its power. Bringing R_e up to 5 kg / cm², that is, to the average value for two-stroke internal combustion engines, we will increase N_emax by 30-35%, having received 38-40 liters. With.

The author has done work to improve this engine. The alteration consisted in the manufacture of four additional purge channels with phases 2-3 ° less than the main ones, a window in the piston and an increase in E_eff. This refinement made it possible to remove 84 kg of thrust on the propeller Ø = 1,08 m, in increments of H = 0,5 m, against 70 kg before the alteration.

According to Table 1, one can also trace the value of the reduction per screw. It is known that the efficiency of the propeller depends on the value of the dynamic pitch:

λ=V/n_c*D, where

V - flight speed, m/s; n_c - the number of revolutions of the screw per second; D - screw diameter, m.

The efficiency of the screw has a maximum at a value of λ=1-1,5; with a larger and smaller value of λ, the efficiency of the propeller drops. This shows that the flight speed and the number of revolutions of the propeller must be in a certain ratio.

Almost half of the engines on the aircraft have a propeller reduction from 0,38 to 0,7, which leads to an increase in static thrust by 80-100%.

Thus, the use of a reduction gear on high-speed motors mounted on low-speed AVS is highly desirable.

Table 1 shows the effect of propeller D on static thrust.

Propeller thrust Р=L a*р*n_c²*D⁴, where a is the thrust coefficient; p is the mass density of air; n_c - number of revolutions of the screw, s; D - screw diameter, m.

Let us dwell briefly on the possible ways of constructive solution of internal combustion engines for ALS. There seem to be two ways. The first is the creation of new engines using the latest advanced technology, with the optimization of the working process parameters; the second is their development on the basis of already existing and proven by long-term practice, through the necessary modification.

The second way is associated with less technical risk and can be carried out in a much shorter time. The starting base for creating engines can be the Whirlwind, RMZ-640, Neptune, and Privet, produced by our industry and widely used by amateurs. These machines are compact, have a small forehead, are dynamically balanced, have a uniform torque and a low speed of rotation of the crankshaft.

Regarding the design features of the engines, it can be noted that the main number of ICEs of the rally (78%) had a crankshaft rotation speed of 5000-6500 rpm, which can be considered optimal. By applying a reduction to the screw 0,4-0,6, it is possible to obtain a compact gearbox (V-belt or simple gear). With an increase in speed, the reduction to the screw increases, which will require a transition to multi-ribbed pulleys due to a decrease in the angle of coverage of the driving pulley for the V-belt drive, which will “pull” an increase in the length and diameter of the propeller shaft console (and, as a result, the weight of the installation) or will necessitate transition to a planetary gear (engine V. Frolov, with n=8000 rpm). The specific gravity of a well-designed and manufactured gear reducer for internal combustion engines of small volumes is 0,14-0,15 kg / l. with., and at high engine speeds, it can "eat" the entire gain in specific gravity.

The author also presents another solution for a two-stroke internal combustion engine for ALS. Keeping in mind that the specific gravity of the engine is inversely proportional to the diameter of the cylinder, it is possible to increase the engine volume to 1,5-2,0 liters by limiting the crankshaft rotation speed within 2400-2600 rpm. Moderate average piston speeds (7-8 m / s) will have a beneficial effect on mechanical efficiency. In such an engine, it is easier to organize gas dynamics, and this will lead to an increase in the filling ratio of the cylinder. The system of direct injection of low pressure fuel will put such an engine on a par with four-stroke machines in terms of specific fuel consumption. The use of non-linered cylinders with nicosil coating or ceramics will further reduce the specific gravity. Such an engine may be lighter than a high-speed ICE of the same power with a gearbox.

In conclusion, we note one more problem posed to the designers of the ALS of future rallies, related to the suppression of exhaust noise. 87% of the rally engines were operated without mufflers. The sound pressure of the exhaust of two-stroke internal combustion engines without a silencer at a distance of 2 m from the exhaust window cut reaches 130-140 dB, which corresponds to the pain threshold. Being under the influence of sound of such power is very tiring and harmful. For two-stroke internal combustion engines, a tuned muffler is even desirable, as it increases power and efficiency.

Based on the above, we can formulate a general approach to the creation of an internal combustion engine for ALS:

• small dimensions,
• low specific gravity g≤0,5 kg/l. With.,
• dynamic balance,
• good throttle response (1-2 sec),
• high profitability, no more than 200 g. l. s/h
• high reliability and durability (1000-1500 h),
• ease of installation and dismantling,
• ease of maintenance,
• low noise level (not higher than 100 d,),
• low unit cost in mass production.

Author: V.Novoseltsev

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