On the left - the crankshaft of the forced "six", on the right - an old motor. The liter capacity has increased very noticeably, and the width of the shaft necks of the “sizing” motor is less! This is a fee for reducing the length of the motor
On the left - the crankshaft of the forced "six", on the right - an old motor. The liter capacity has increased very noticeably, and the width of the shaft necks of the “sizing” motor is less! This is a fee for reducing the length of the motor! On the left - the crankshaft of the forced "six", on the right - an old motor. The liter capacity has increased very noticeably, and the width of the shaft necks of the “sizing” motor is less! This is a fee for reducing the length of the motor!
The time of engines that passed from grandfather to grandson was almost over. Like the era of enormous mastodons who consumed tons of fuel … It is obvious that one is closely connected with the other.
The main modern trend in engine building is to cram the maximum "number of horses" into as little hood space as possible. And this is understandable: such a miniaturization of power units has many advantages. The compact wagon acquires previously unattainable energy saturation and fits the latest requirements in ecology.
And what have to sacrifice for this?
MARES IN LITER
The degree of forcing (just like that: “forcing” is considered slang!) Of the motor is one of the most important parameters of its perfection. At the same time, its absolute power does not, in general, speak of anything, since the truck engine is 450 hp. it can be much less forced than the two-hundred-horsepower engine of a modern “passenger car”.
To characterize the degree of forcing, two quantities are introduced in the classical theory of ICE. One - rather difficult to understand - is called "average effective pressure": this is the ratio of useful work to engine displacement. About thirty years ago, for automobile engines, this figure ranged between 0.6 … 0.8 MPa. And now modern high-speed turbocharged engines climb to the level of 1.4 … 1.6 MPa. By the way, some diesel engines are already designing for an average effective pressure of 2.5 … 3.0 MPa. But this is a complex parameter that is used only among specialists. In the descriptions of motors on the "sites" and in the "letters" you will not find it.
The “liter power” obtained by simply dividing the rated motor power by its working volume is more obvious. And here progress is clearly visible. Motors of the Moscow Olympics era produced 40 … 50 "horses" per liter of working volume. Now we have reached the cherished 100 hp / l. Such numbers in the old days were only in "very sports" engines.
TRENDS AND COMPROMISES
1 no copyright
Forcing is an increase in power. And with it, an increase in pressure in the cylinder. Consequently, all mechanical loads increase in proportion. And we remove strength by reducing size …
So, two trends. On the one hand, we reduce the size of the engine and its parts. At the same time, we understand that strength decreases with decreasing size - there are no miracles! On the other hand, we increase the degree of forcing, thereby increasing the load on the parts! Moreover, the load is both thermal and mechanical. The first ones will grow approximately in the same way as the amount of fuel supplied to the cylinders increases … But with mechanical things are generally difficult: you can force the engine in different ways: by boost and by turns. In the first case, gas pressure increases, in the second - inertial loads. Both have different effects on the strength of the structure and its resource.
By the way, the reduction in the size of some parts, in particular, pistons and connecting rods, is more connected not with a decrease in the overall size of the engine, but with the desire to reduce their masses, thereby reducing the inertial loads on the bearings of the crankshaft and the skeleton of the engine. This is done to reduce friction losses in the motor and, as a result, directly affects both its efficiency and CO2 emissions.
So - two mutually exclusive trends that guarantee a simultaneous decrease in strength and increased loads. Therefore, a compromise must be sought. But it is already obvious that his search leads to where we started - reducing the life of the motor. And this is inevitable.
2 no copyright
With the forcing and miniaturization of parts, the loads on the shaft necks increase, and therefore the resource and reliability fall …
It’s time to figure out how the load of the “sizing” motor is growing. Maybe it's not so scary?
To do this on a real stand is hard: too expensive, long and difficult. Theoretical calculations also require initial data that no company will provide: intellectual property, you know! Therefore, we will do it easier: we compare three options for engines of one reputable company. The first is relatively old, unpretentious, with a working volume of 1.6 liters and a capacity of about 100 hp, that is, with a liter capacity of 63 hp / l. This motor has earned a reputation as an almost indestructible, as it lasts longer than the car itself. We will take it as a base for comparison. Two other motors are modern, forced. One is an in-line 2-liter Quartet delivering 200 hp. The other is a V-shaped 3-liter “six”, 280 hp. Here, the liter capacity is already very substantial - under a hundred forces per liter with very modest dimensions. Naturally turbocharged engines with direct fuel injection.
Measurement of details immediately made me think. With almost the same diameter of the cylinders and a close diameter of the necks of the crankshaft, the lengths of the main necks of both the old and the new engines are almost the same, about 24 mm. The connecting rods of the in-line “fours” are also close - 22 mm, but in the 3-liter “six” they are narrower - only 17 mm. In principle, the layout is clear: two rods should be placed on one neck. But what about the loads?
Here I had to turn to modern methods of modeling processes in the engine. The results are on the graphs!
Naturally, the boost gave not only an increase in power, but also a significant increase in pressure in the cylinder. Yes, it’s not without reason that they are asking for the 98th gasoline for turbocharged engines! Expensive, but what to do - love to ride, love and sled (with money) to carry! But the main thing is that the loads on the shaft journals have increased dramatically! Compared to the base motor, the crank pin loads for the 2-liter engine increased almost in proportion to the increase in boost - by 30 … 35%. But for a 3-liter engine, the increase in the load on the connecting rod journals was even more pronounced, even despite the lower level of liter power - almost 45%! The decrease in the width of the neck affected - it could not be otherwise!
Loads are the wear rate of the crankshaft bearings. We will evaluate this as well. The oil was taken one and the same, good “synthetics” 5W-40. The mode is also only one - nominal, for which previous assessments were made. So, the wear rate of the bearings, according to our calculations, with the same installation clearances increased by 50 … 60% compared to the base engine! Increased loads and increased temperatures led to an increase in the length of the zones of violation of the integrity of the oil film, where wear and tear lives. For cylinders and rings, the picture is more humane, but the increase in wear is noticeable here too - the same loads and temperatures do their job. But today it’s fashionable to make motors completely aluminum, and use low-viscosity oils - so the figures obtained for reducing the resource can still be considered optimistic.
Where is the piston and connecting rod of the forced motor? That's right - down below! But in life, grace is the virtue of weakness and defenselessness
Where is the piston and connecting rod of the forced motor? That's right - down below! But in life, grace is the virtue of weakness and defenselessness. Where is the piston and connecting rod of the forced motor? That's right - down below! But in life, grace is the virtue of weakness and defenselessness.
Loads, wear … But that's not all! Remember the trade-off between strength and size? Typically, when designing a motor, reliability is ensured by the fact that large safety factors are laid in the design of parts - no less than one and a half along the crankshaft and connecting rod. They provide "foolproofness" of the structure, that is, its non-destruction during short-term overload above the nominal value or other emergency situation. And the decrease in strength associated with a decrease in the size of the parts, simultaneously with an increase in the load, reduces these safety margins to almost the minimum values … Our estimates have fully confirmed that for forced motors along the crankshaft and connecting rod the safety margins did not exceed 5 … 10%. What are these numbers talking about? Only that the reliability of the structure as the ability to maintain its operability in critical situations fell victim to downsizing. We are not talking about the motor resource anymore.
LONG LONG LIVERS
But what about the new materials? New technologies? They compensate for the increase in loads!
Theoretically, yes. But, firstly, partially. Secondly, as our experience with car factories shows, few people tend to use more expensive materials in mass production - in practice, it drives the cost … And low cost is the main enemy of quality: this is the law. Therefore, talk about the systematic limitation of the service life of cars in general and engines in particular is quite justified. And the eternal car is no longer needed by the manufacturer.