Thus, the thermal efficiency of internal combustion engines of small size is relatively low and specific fuel consumption they will be higher.
Table 1 shows the dimensions of the cylinder, the piston and its relative course S/D. These parameters are closely linked, so we will consider them together.
Almost all the engines referred to have the relative move is less than one, and short-stroke engines have several advantages over clinohedrite: here and the possibility of placement of channels of large cross section, which increases the filling of the cylinder; and a decrease in the average speed of the piston, thereby increasing mechanical efficiency. Finally, short-stroke engine more compact dlinnonogih.
The next factor is the speed of the piston
S —piston stroke, m; n — frequency of crankshaft rotation, Rev/min. The average speed of the piston engines shown in the table, from 8.4 m/s to 17 m/s. This indicator seriously affects the dynamic load of the parts of the engine, filling the cylinder and the amount of energy expended on the friction of the pistons and bearings. Mean piston speed special motors for ALS 12-15 m/s.
The frequency of rotation of the crankshaft (see table 1) under consideration of power plants — from 4500 rpm to 8000 rpm. it is Known that the power of the engine depends on its specific speed. However, accompanied by a sharp force (proportional to the square of the speed) is increased, the inertia forces of the rotating and translational moving mass of the engine parts and, as a consequence, the increase in friction loss, which requires increased mechanical strength of the parts of the engine and the change of the bearing working conditions. On the other hand, the increase in speed is limited by the cooling of the cylinder head, piston, spark plug, since an increase in rpm increases the heat removal from the cylinder. In addition, the speed of rotation is limited to average speed of the piston, which increases pressure loss in the purge increase dramatically (in proportion to the square of the speed of the piston), which reduces filling and reduces engine power. However, the speed increase up to a certain limit improves η
Table 1 shows also the mean effective pressure and compression ratio. From the formula of power shows that there are two main directions of increase power is to increase the speed and pressure P
e. The effect of speed on power we’ve seen so far. See how you can improve R e.
This is easily achieved by increasing the E — compression ratio (for two-stroke engines applies effective compression).
EF=(V EF+V KS)/V KS, where
E EF is the effective volume described by the piston from the top edge of the exhaust window before TDC, V KS — combustion chamber volume (see table. 3).
Graph of the effect of compression ratio (solid line) and boost (dashed lines), pressure at end of combustion. P z and the specific fuel consumption C e (%).
This method is good because it is simple and, except to increase power, reduces fuel consumption. However, it has disadvantages.
The increase in E is accompanied by increasing the temperature and pressure at the end of the compression stroke, causing a sharp rise in combustion pressure P
e, and consequently is a need for more durable parts, tightens the requirements for fuel and oil. However, the effect of increasing energy from the increasing R e is the physical border more than 15-20%, so the power does not increase. When compression ratios 10-12 power increase is already negligible. Up to what limit it is possible to increase the compression ratio from the point of view of the practical benefits? The rise of P z and η t can be traced with an increase from 4 to 8. Omitting the design side, here is the result.
The compression ratio E, equal to 4, 5, 6, 7, 8, correspond to the combustion pressure P
z 25.3 kg/cm 2, 34 kg/cm 2, 44,0 kg/cm 2, 54,2 kg/cm 2 and 65.5 kg/cm 2. This shows that increasing from 7 to 8 we will win in the efficiency η t is only 4.6%, whereas the combustion pressure is increased from 54,2 to 65.5 kg/cm , i.e. 20%. Therefore, in practice, we need to compromise between optimal compression ratio and η t (see chart).
For practical use we can recommend the most favorable value of the degree of compression when working on fuel, not detonating at all times.
Another method of increasing R
e is to increase the pressure of the mixture at the inlet.
From two-stroke engines increase R
e is achieved by using the resonant pipe on the suction and exhaust (effect of Cadence, which he discovered in 1903 and first implemented on the engine of the company “YuMO” in 1923, when it was received a capacity increase of 60%). Tuned exhaust system, for example, increases the power up to 30-40% without much increasing the mass of the motor, besides improving its efficiency.
The increase in R
e have four-stroke engines is associated with considerably greater difficulties. Even a simple change of valve timing will put the designer in front of a serious technological and design challenges of manufacturing camshaft, bore the seats and install new valves, etc.
Our statistics gives the following R
e: for four-stroke internal combustion engine from 9.5 to 10 kg/cm 2, two-stroke have from 3.6 to 6.6 kg/cm 2, 40% two-stroke engines R e ranges from 5.1 to 6.5 kg/cm 2, which is a good indicator. However, the engine RMZ-640 (one of the most common at the rally) R e is only 3.6 kg/cm 2, indicating reserves of increase of its power. Bringing R e to 5 kg/cm 2, that is, to the average values for the two-stroke engine, we increase N e max 30-35%, getting 38-40 HP
The author of the work has been done to improve this engine. Alteration was the manufacture of four additional vent channels with phases 2-3° less than the main Windows in the piston and the increase of E
EF. This improvement allowed to withdraw 84 kg of thrust on the screw Ø = 1,08 m with a step of H=0.5 m, vs 70 kg to alteration.
In table 1 it is possible to trace the importance of the reduction on the screw. It is known that the efficiency of the propeller depends on the magnitude of dynamic steps:
V — airspeed, m/s;
n c is the number of propeller revolutions per second; D is the screw diameter, m.
The efficiency of the propeller has a maximum at the value λ=1-1,5; with a larger and a smaller value of λ the efficiency of the propeller decreases. This shows that the flight speed and the number of revolutions of the screw must be in a certain ratio.
In modern high speed engines the efficiency of the propeller decreases dramatically to 0.3 to 0.5, by reducing the dynamic step, especially when installing the motor on low-speed aircraft. Therefore, it appears advantageous to bring the screw from the crankshaft and through a reduction gear.
Almost half of the engines on SLA is the reduction of the screw from 0.38 to 0.7, which leads to higher static thrust at 80-100%.
Thus, application of the reduction gearing for high speed motors installed on low-speed SLA, is highly desirable.
Table 1 shows the effect of the D screw on the static thrust.
The thrust of the screw R=L a*R*n
c 2*D 4where a is the coefficient of thrust; p — mass density of air; n c is the number of revolutions of the screw; D is the screw diameter, m.
It is seen that the gain in thrust by increasing the propeller diameter is obtained significant. For example, increased D by 5% and increases thrust by 21% and 10% gives a rise of 46%.
Will briefly describe possible ways of constructive solutions of DVS for ALS. There are two ways. The first is the development of new engines using the latest advanced technologies, with optimization of workflow parameters; the second is to develop them on the basis of already existing and tested by long practice, by necessary modifications.
The first way will give the best results, but require large material costs, research and theoretical work. And the timing of the creation of such internal combustion engines will be high, since the technical production of aircraft piston engines largely lost with the transition to the gas turbine.
The second path is associated with less technical risk and could be implemented in significantly less time. The starting point for creating engines can serve as produced by our industry and widely used by lovers of “Vortex”, RMZ-640, “Neptune”, “Hello”. These machines are compact, have a small “head” that is dynamically balanced, has an even torque and low rotation speed of the crankshaft.
Referring to the particular construction of the engines, it can be noted that the main number of the internal combustion engine of a meeting (78%) had a rotational speed of the crankshaft 5000-6500 rpm, which can be considered optimal. Applying the reduction to the screw of 0.4—0.6, it is possible to obtain a compact gear reducer (V-belt or a simple gear). With increasing rapidity grows reduction on the screw that will require transition at multiple pulleys due to the reduction of the angle of coverage of a driving pulley for V-belt transmission that will draw in an increase of the length and diameter of the screw shaft of the console (and as a consequence — the weight of the installation) or will necessitate the transition to the planetary gear (engine V. Frolov, n=8000 rpm). The specific gravity of a properly designed and fabricated gear for internal combustion engines of small volumes is 0.14—0.15 kg/HP, and at high engine speeds, it can “eat” the entire gain on mass.
The author seems to be another solution of the two-stroke engine for ULTRALIGHTS. Remembering that the weight of the engine is inversely proportional to the diameter of the cylinder, you can increase the volume of motor up to 1.5—2.0 l, limiting the rotational speed of the crankshaft in the range of 2400-2600 rpm. Moderate average piston speed (7-8 m/s) have a beneficial effect on mechanical efficiency. This engine is easier to organize the dynamics, and this will lead to an increase of the filling ratio of the cylinder. The system of direct fuel injection low pressure will put this engine in a row with a four-stroke machines in specific fuel consumption. Application devilsbane cylinders and nikosilver coating or ceramics will further reduce the proportion. This engine can be lighter than a speed of the internal combustion engine of the same capacity with gear.
In conclusion, we note another problem posed to designers of future SLA meetings associated with the silencing of the exhaust noise. 87% of the fleet of engines of the rally operated without mufflers. Sound pressure exhaust two-stroke engine without a muffler at a distance of 2 m from the edge of the exhaust window reaches 130 to 140 dB, which corresponds to the pain threshold of feeling. To be under the influence of this power is very tiring and harmful. For two-stroke engine tuned muffler even desirable, as it improves the economy and power.
Based on the review it is possible to formulate a General approach to the creation of DVS for ALS:
— small overall dimensions,
— low specific weight g≤0.5 kg/HP,
— dynamic balance,
— good throttle response (1-2 sec),
— high efficiency, not more than 200 g. l/h
— high reliability and durability (1000-1500 h),
— easy mounting and Dismounting,
— easy maintenance
— low noise level (not exceeding 100 d),
— low unit cost in mass production.