NON-PROFIT EDUCATIONAL INSTITUTION "RUSSIAN TECHNICAL SCHOOL"

"INTERNAL COMBUSTION ENGINE"

"Engine specifications."

The main characteristics of the engine include power, torque and fuel efficiency.

Engine power.

In an internal combustion engine, the gas pressure resulting from the combustion of the air-fuel mixture acts on the piston crown and moves the piston in the cylinder. By moving the piston, the gases do useful work*, and the engine develops a certain power**.

*Work(A) occurs when a force (F) acts on the body and under the influence of this force the body moves (moves a distance S). In other words: Mechanical work is directly proportional to the applied force and the distance covered (A=FS). The SI unit of measure for work is Joule(J). One joule equals one Newton, multiplied by one meter (1J = Nm), i.e., if a force of one Newton moves a body with a mass of one kg over a distance of one meter, then such a force is equal to one Joule.

**Power(P) is equal to the work (A) done in a certain time (unit of time - t): P \u003d A / t (Power \u003d Work / Time). The unit of power in the SI system is Watt(W). One Watt is equal to one Joule divided by one second (1W=1J/1sec), i.e., if work of one Joule is done in one second, then such work reproduces a power equal to one Watt. An off-system unit of power measurement is a kilogram-force multiplied by one meter divided by one second (kgf m / s). 1kgf m/s = 9.81W. The technical literature on automotive topics also uses such a unit of measurement as horsepower. One horsepower is equal to 75 kgf m / s and 735.5 watts.

The power developed by the gases inside the engine cylinders is called indicator power (P i ). The indicator power cannot be fully used to move the car, since part of this power is spent on overcoming friction forces in the engine itself (friction in bearings, between parts of the cylinder-piston group and the gas distribution mechanism, oil agitation, etc.), as well as driving auxiliary mechanisms (generator, coolant pump, etc.).
The power that can be taken from crankshaft engine and used to move the car is called effective power ( R ef).
Effective power less indicator power on the amount of mechanical losses. It is convenient to represent mechanical losses in the form of a mechanical Engine efficiency (η).
Engine efficiency is equal to the ratio of effective and indicated power ( η = R ef / P i ). Efficiency value modern engines lies within 0.7 - 0.9. The value of efficiency is determined experimentally on special installations ( brake installations drum or other type, developing a given braking force).
The effective power of the engine is described by the formula: R ef= p i V d n/2x60x75 (hp), where in the numerator:
p i - average indicator gas pressure (kg / sq.m.) acting on the piston;
V d - engine displacement (m3);
n- the number of engine revolutions (rpm);
in the denominator:
2 - numerical coefficient (for four-stroke engines = 2, for two-stroke engines = 1);
60x75 - numerical coefficient for converting the power value from "kgf m / min" to " horsepower».

It follows from the formula that the effective power of the engine depends on: 1) the average indicator pressure of gases acting on the piston, 2) the working volume of the engine and 3) the number of work cycles carried out during the conditional time of the engine, expressed in revolutions of the crankshaft.

Average indicated gas pressure (p i ) is a conditionally constant pressure which, acting on the piston during one working stroke, does work equal to the indicator work of gases in the cylinder during the working cycle, i.e. p i = BUT i / V c (the ratio of the indicator work of gases BUT i to unit cylinder displacement V c).
Average indicated pressures at rated load for four-stroke gasoline engines 0.8 - 1.2 MPa, for four-stroke diesel engines 0.7 - 1.1 MPa, for two-stroke diesel engines 0.6 - 0.9 MPa.

Engine displacement V d is equal to the sum of the working volumes of all its cylinders ( V d = Σ n V c). The working volume of one cylinder ( V c ) is equal to the product of its diameter (d) and the piston stroke (h) - ( V c = dh).

Number of working cycles performed by the engine in one minute is equal to 2n/T, where n- frequency of rotation of the crankshaft, T- cycle rate of the engine (the number of cycles performed per working cycle). For a four-stroke engine, T = 4, and the number of work cycles is n/2.

Of the above values, constants, i.e. unchanged, depending on the design of the engine, are only the displacement and engine cycle. The rest are variables. The values ​​of these quantities will depend on the mode of operation and the technical condition of the engine. It can be seen from the formula that with an increase in the crankshaft speed and the pressure of the gases acting on the piston, the engine power will also increase. At the same time, the function of power from the speed of rotation of the CV is not linear, which is illustrated in the graph (Fig. 1).

This fact requires some explanation.
The fact is that the pressure of the working gases depends on the completeness of filling the cylinders with a new portion of the air-fuel mixture, the speed and completeness of its combustion and the degree (coefficient) of the subsequent cleaning of the cylinders from exhaust gases. The degree of filling and cleaning of the cylinders, as well as the speed and completeness of the combustion of the air-fuel mixture, are determined by the design and setting of the gas distribution mechanism, intake and exhaust systems, fuel system, as well as the algorithm for the operation of fuel supply, ignition, air boost and valve timing control systems and is only slightly related to the speed of rotation of the crankshaft. The maximum power is developed by the engine upon reaching such values ​​of crankshaft speed, which will correspond to the optimal settings and performance of the listed systems and mechanisms, providing the necessary conditions mixture formation, mixture combustion and cylinder cleaning. In all other cases (revs higher or lower), the engine performance will be below the maximum values.
In the technical literature, the speed at which the maximum declared engine power is reached is referred to as " maximum power revolutions».
Engines whose maximum power is achieved at high crankshaft speeds (5000 rpm or more) are called high-speed(high speed). Engines whose maximum power is reached at low crankshaft speeds (less than 5000 rpm) are called slow-moving(low speed). From the point of view of consumer interest in the products of the automotive industry, it is very simplified, but we can say that the power performance of the engine determines the speed properties of the car. That is, a high-speed engine, ceteris paribus, will provide better speed characteristics of the car than a low-speed engine. Max speed the car will reach its maximum power at revs. When the engine reaches the maximum power mode, the engine starts to work only to overcome the forces of resistance to movement, the car does not accelerate.

For comparative evaluation various engines from the point of view of the perfection of the work process and design, they use the value " liter capacity". Liter power is equal to the ratio of engine power to its working volume ( P L= P ef / V d). This value shows how much power can be "removed" from one liter of engine displacement. The greater the liter power, the smaller the relative dimensions and specific gravity of the engine, ceteris paribus, the higher its technical and design indicators. The liter power of modern engines lies in the range of 15 - 37 kW / l - for gasoline engines, and 6 - 22 kW / l - for diesel engines.

Torque

When the engine is running, a torque develops on its crankshaft, which is transmitted through the transmission mechanisms to the driving wheels of the car and sets the car in motion. Torque ( M k ) is equal to the product of the force ( F) on the shoulder of its action ( r) and is measured in newtons multiplied by a meter ( H x m) or in kilogram force multiplied by a meter (kgf x m).
Mk=F x r;
In the engine, the force of action is the pressure of the gases. The arm of the force is the crankshaft crank. The higher the pressure of the gases acting on the piston, and the larger the radius of the crank, the greater the torque the engine develops. The pressure value of the working gases depends on a number of conditions discussed in the previous subsection (Engine power). The crank radius is determined by the design of the engine.
The engine torque increases with an increase in crankshaft speed and reaches its maximum value at the so-called. "revs of maximum torque". crankshaft revolutions corresponding to the maximum torque revolutions for different types engines are in the range of 1500 - 3000 rpm (diesels) and 3000 - 4500 rpm (gasoline engines). The "binding" of the maximum torque to the crankshaft speed, as in the case of power, is due to the adjustment of the gas distribution mechanism of the motor of its intake and exhaust tract, as well as the power supply and engine control system.
Engine power and torque are related by the formula: M k = 716.2 P ef / n(kgf m);
The torque is transmitted by the transmission to the driving wheels of the car and determines the traction force of the driving wheels: F t = M k x c x η /r, where F t is the thrust force; M k is the torque; c- the total gear ratio of the transmission; η - transmission efficiency (0.88 - 0.95); r is the radius of the driving wheels.
From the point of view of consumer interest in automotive products, it is simplified, but we can say that the torque determines the traction characteristics of the car. The more torque the engine develops, the higher the traction on the drive wheels. Fast growth engine torque indicates a good acceleration dynamics of the car due to an intensive increase in the traction force of the drive wheels.
The longer the value of the moment is in the region of its maximum and does not decrease, the better engine adapted to change road conditions(the less often you have to change gears).
Low-speed motors have large torques.

Fuel economy

Economy of work car engine is measured by the amount of fuel in grams consumed for each unit of power per unit of time (one hour) and is called " specific fuel consumption» ( g e g/kWh). Fuel consumption increases with an increase in crankshaft speed and depends on the perfection of the engine design and its technical condition. Total(total) fuel consumption is characterized by fuel consumption in kilograms per hour of work and is called " hourly fuel consumption» ( G T kg/h). The specific fuel consumption can be determined by the formula g e= G T 1000/ P ef (g/kWh).

How can the same engine have different output? What is the difference between power and torque?

WHAT IS HORSEPOWER?

How much strength do you have? - such a question was heard by anyone who touched the world of cars at least a little. No one even needs to explain what forces are actually meant - horses. It is in them that we are accustomed to assessing the power of the motor, one of the most important consumer characteristics of the car.

There is practically no horse-drawn transport left even in the villages, and this unit of measurement has been alive and well for more than a hundred years. But horsepower is, in fact, illegal. It is not included in the international system of units (I think many people remember from school that it is called SI) and therefore does not have an official status. Moreover, the International Organization of Legal Metrology calls for the removal of horsepower from circulation as soon as possible, and the EU directive 80/181 / EEC of January 1, 2010 directly obliges car manufacturers to use traditional "hp" only as an auxiliary value for indicating power.

But it is not in vain that habit is considered second nature. After all, we say in everyday life “copier” instead of copiers and call adhesive tape “adhesive tape”. Here are the unrecognized "hp" Now not only ordinary people use it, but almost everyone automotive companies. What do they care about advisory directives? If it is more convenient for the buyer, so be it. Why are there manufacturers - even the state is on about. If anyone has forgotten, in Russia the transport tax and the OSAGO tariff are calculated from horsepower, as well as the cost of evacuating improperly parked vehicles in Moscow.

And no one would find fault with this, if not for one significant “but”. Conceived to make life easier for us, horsepower is actually confusing. After all, it appeared in the era of the industrial revolution as a completely conditional value, which, not only to an automobile engine, even to a horse, has a rather indirect relationship. The meaning of this unit is as follows - 1 hp. enough to lift a 75 kg load to a height of 1 meter in 1 second. In fact, this is a highly average performance indicator for one mare. And no more.

In other words, the new unit of measurement was very useful to industrialists who extracted, for example, coal from mines, and manufacturers of related equipment. With its help, it was easier to assess the advantage of mechanisms over animal strength. And since the machines were already driven by steam, and later by kerosene engines, the “hp” passed by inheritance to self-running crews.

Ironically, the man who invented the horsepower is the official unit of power, James Watt. And since the watt (or rather, in relation to powerful machines, kilowatt - kW) was also actively used by the beginning of the 19th century, the two quantities had to be somehow brought to each other. This is where the key disagreements arose. For example, in Russia and most other European countries they adopted the so-called metric horsepower, which is equal to 735.49875 W or, which is more familiar to us now, 1 kW = 1.36 hp. Such "hp" most often denoted by PS (from the German Pferdestarke), but there are other options - cv, hk, pk, ks, ch ... At the same time, in Great Britain and a number of its former colonies they decided to go their own way, organizing an "imperial" measurement system with its pounds, feet and other delights, in which mechanical (or, in other words, indicator) horsepower was already 745.69987158227022 watts. And then - off we go. For example, in the USA they even came up with electric (746 W) and boiler (9809.5 W) horsepower.

So it turns out that the same car with the same engine in different countries on paper can have different power. Take, for example, our popular crossover Kia Sportage- in Russia or Germany, according to the passport, its two-liter turbodiesel in two versions develops 136 or 184 hp, and in England - 134 and 181 "horses". Although in fact the output of the motor in international units is exactly 100 and 135 kW - and anywhere in the world. But, you see, it sounds unusual. And the numbers aren't that impressive anymore. Therefore, automakers are in no hurry to switch to an official unit of measurement, explaining this by marketing and traditions. How is that? Competitors will have 136 forces, but we only have 100 kW? No, that will not do…

HOW IS POWER MEASURED?

However, "powerful" tricks are not limited to playing with units of measurement. Until recently, it was not only designated, but even measured in different ways. In particular, in America for a long time (until the early 1970s), automakers practiced bench tests of engines stripped naked - without a hitch like a generator, an air conditioning compressor, a cooling system pump, and with a straight-through pipe instead of numerous mufflers. Of course, the motor that threw off the fetters easily gave out 10-20 percent more "hp", so necessary for sales managers. Indeed, few of the buyers went into the intricacies of the test methodology.

Another extreme (but much closer to reality) is taking indicators directly from the wheels of a car, on running drums. This is what racing teams, tuning shops and other teams do, for which it is important to know the return of the engine, taking into account all possible losses, including transmission losses.

But in the end, a compromise was accepted as a sample in various methods like European ECE, DIN or American SAE. When the engine is installed on a stand, but with all the hitch necessary for smooth operation, including a standard exhaust path. You can only remove equipment related to other machine systems (for example, an air suspension compressor or a power steering pump). That is, they test the motor exactly in the form in which it actually stands under the hood of the car. This allows us to exclude the "quality" of the transmission from the final result and determine the power at the crankshaft, taking into account the losses on the drive of the main mounted units. So, if we talk about Europe, then this procedure is regulated by Directive 80/1269 / EEC, first adopted back in 1980 and since then regularly updated.

WHAT IS TORQUE?

But if power, as they say in America, helps cars sell, then torque moves them forward. It is measured in newton meters (N∙m), but most drivers still do not have a clear idea about this characteristic of the motor. At best, ordinary people know one thing - the higher the torque, the better. Almost like power, isn't it? That's just how then "N∙m" differ from "hp".?

In fact, they are related quantities. Moreover, power is a derivative of the torque and speed of the motor. And it is simply impossible to consider them separately. Know - to get the power in watts, you need to multiply the torque in Newton meters by the current number of revolutions of the crankshaft and a factor of 0.1047. Want familiar horsepower? No problems! Divide the result by 1000 (thus you get kilowatts) and multiply by a factor of 1.36.

In technical terms, power refers to how much work a motor can do per unit of time. But the torque characterizes the potential of the engine to perform this very work. Shows the resistance he can overcome. For example, if the car rests its wheels on a high curb and cannot move, the power will be zero, since the motor does not perform any work - there is no movement, but the torque develops. After all, in that moment, until the engine stalls from strain, the working mixture burns out in the cylinders, the gases put pressure on the pistons, and the connecting rods try to put the crankshaft into rotation. In other words, moment without power can exist, but power without moment cannot. That is, it is “N∙m” that is the main “product” of the engine, which it produces by converting thermal energy into mechanical energy.

If we draw analogies with a person, “N∙m” reflects his strength, and “hp” - endurance. That is why slow moving diesel engines by virtue of their design features we usually have weightlifters - ceteris paribus, they can drag more on themselves and more easily overcome resistance on wheels, albeit not so quickly. But high-speed gasoline engines are more likely to be runners - they hold the load worse, but they move faster. In general, there is a simple rule of leverage - we win in strength, we lose in distance or speed. And vice versa.

How is this expressed in practice? First of all, you need to understand that it is the torque and power curves (together, not separately!) on the so-called external speed characteristic of the engine that will reveal its true capabilities. The earlier the thrust peak is reached and the later the power peak is reached, the better motor adapted to their tasks. Let's take a simple example - a car is moving on a flat road and suddenly it begins to rise. The resistance on the wheels increases, so that with the same fuel supply, the speed will begin to fall. But if the engine characteristic is competent, the torque will, on the contrary, begin to grow. That is, the motor itself will adapt to the increase in load and will not require the driver or electronics to switch to a lower gear. The pass is passed, the descent begins. The car started accelerating - high traction is no longer so important here, another factor becomes critical - the motor must have time to produce it. That is, power comes to the fore. which can be regulated not only gear ratios in the transmission, but by increasing the engine speed.

Here it is appropriate to recall racing car or motorcycle engines. Due to the relatively small working volumes, they cannot develop a record torque, but the ability to spin up to 15 thousand rpm and above allows them to produce fantastic power. For example, if a conventional engine at 4000 rpm provides 250 N∙m and, accordingly, approximately 143 hp, then at 18000 rpm it could already produce 640.76 hp. Impressive, isn't it? Another thing is that "civilian" technologies do not always manage to achieve this.

And, by the way, in this regard, electric motors have a characteristic that is close to ideal. They develop maximum "Newton meters" right from the start, and then the torque curve smoothly drops with increasing speed. At the same time, the power graph progressively increases.

I think you already understood - in the characteristics of a car, not only the maximum values ​​\u200b\u200bof power and torque are important, but also their dependence on revolutions. That is why journalists are so fond of repeating the word "shelf" - when, for example, the engine produces a thrust peak not at one point, but in the range from 1500 to 4500 rpm. After all, if there is a supply of torque, power is also likely to be missed.

But still, the best indicator of the "quality" (let's call it that) of the return of an automobile engine is its elasticity, that is, the ability to gain momentum under load. It is expressed, for example, in acceleration from 60 to 100 km / h in fourth gear or from 80 to 120 km / h in fifth - these are standard tests in automotive industry. And it may happen that some modern turbo engine with high traction at low revs and a wide torque shelf gives a feeling of excellent dynamics in the city, but on the highway when overtaking it will turn out to be worse than the ancient aspirated engine with a more favorable characteristic of not only torque, but also power ...