The abbreviation VTEC is fully deciphered as follows – Variable Valve Timing and Lift Electronic Control. This mechanism is designed to optimize the flow of the air-fuel mixture into the combustion chambers.
The combustion engine converts the chemical energy stored in the fuel into thermal energy. This conversion takes place during the combustion of the combustible mixture. This increases the temperature and pressure in the cylinder. Under pressure, the engine pistons move downward and push the crankshaft into motion. This is how chemical energy is converted into mechanical movement. The mechanical force is determined by the amount of torque. The ability of an engine to maintain some amount of torque at some number of revolutions per minute is defined as power. Power determines how much work an engine can produce. The entire process performed by an internal combustion engine is not 100% efficient. In fact, only about 30% of the energy in the fuel is converted into mechanical energy.
Theoretical physics tells us that at this efficiency, more fuel must be used to achieve high output from the motor: the result will be a significant increase in power. It is obvious that in this case, it is necessary to use an engine with a huge working volume and to compromise the principles of the economy. The other method dictates that the fuel mixture must be pre-compressed by means of a turbine and then burned in small cylinders. But even in this case, the fuel consumption will be frightening. At one time, Honda took a different route, starting research to optimize the combustion engine. The VTEC technology appeared as a result, which provides the engine with an excellent economy at low rpm and high power when “cranking” it.
How VTEC Works
Two algorithms
If you compare the speed characteristics of different engines, it is easy to see that some engines reach maximum torque at low revolutions (in the range of 1800-3000 rpm), others – at higher (in the range of 3000-4500 rpm). It turns out that there is a correlation between the way the cams that open the valves are mounted on the camshaft and how much power the engine develops at various crankshaft speeds. To understand what causes this, imagine an engine running extremely slow. For example, at 10-20 revolutions per minute, the duty cycle in one cylinder takes 1 second. As the piston lowers, the intake valve opens, allowing the combustible mixture to fill the cylinder, and closes when the piston reaches the bottom dead center. When the combustion cycle is complete, the piston will begin to move upward. This will open the exhaust valve, allowing the exhaust gases to leave the working volume of the cylinder, and close when the piston reaches the top dead center. Such an algorithm would be ideal if the engine were running at minimum rpm. In real life, however, the engine is much more vigorous.
As the rhythm of the engine increases, the described algorithm simply does not stand up to criticism. If the crankshaft rpm reaches 4000 per minute, the valves open and close 2000 times every minute, or 30-40 times every second. At this speed, it is extremely difficult for the piston to suck the necessary amount of combustible mixture into the cylinder. That is, pumping losses occur as a result of intake resistance, and this is the main reason why engine efficiency is reduced. To make it easier for the engine to run at higher rpm, the intake valve, for example, has to be opened wider. Of course, this is a simplified description of operation, but it gives a general idea. However, such an algorithm is no good at low revs: adjusting the camshaft “for speed” will only increase fuel consumption. Consequently, for better efficiency, it is necessary to combine both operating algorithms, which are embodied in the VTEC mechanism.
Introduced in 1989, the VTEC system has been upgraded twice, and today we are dealing with its third series. The VTEC system takes advantage of electronics and mechanics and allows the engine to effectively manage the capabilities of two camshafts at once, or, in simplified versions, one. By controlling the engine’s RPM and operating ranges, its computer can activate additional cams to select the best operating mode.
DOHC VTEC.
In 1989, two modifications of the Honda Integra, the RSi and XSi, entered the Japanese domestic market using the first engine with the DOHC VTEC system. Its power unit model B16A with a capacity of 1.6 liters has reached 160 horsepower, but at the same time is characterized by good traction at low levels, fuel efficiency, and environmental friendliness. Fans of the Honda brand still remember and appreciate this great engine, especially since its repeatedly improved version is still used on the Civic models today.
The DOHC VTEC engine has two camshafts (one for the intake valves and one for the exhaust valves) and four valves per cylinder. For each pair of valves, there is a special design – a group of three cams. Consequently, if we are dealing with a four-cylinder 16-valve engine with two camshafts, there will be 8 such groups. Each group deals with a different pair of valves. Two cams are located on the outer sides of the group and are responsible for valve action at low rpm, and the middle one is connected at high rpm. The outer cams are in direct contact with the valves: they lower them with rocker arms (rockers). The individual middle cam for the time being rotates and idles on its rocker arm, which activates when a certain high crankshaft speed is reached. This center part is then responsible for opening and closing the valves, although it acts as a special intermediate mechanism.
When the engine is running at low rpm, the intake and exhaust valve pairs are opened by their respective cams. Their shape, as with most similar engines, is elliptical. These cams, however, can only provide economical engine operation and only at low rpm. When the camshaft reaches high speed, a special mechanism is engaged. “Unoccupied” by this work, the middle cam rotates and without any effect on the middle rocker, which is in no way connected to the valves. However, all three rocker arms have holes into which a metal bar is driven under high oil pressure. Thus, the group is rigidly fixed and subsequently works as one unit. This is where the previously resting middle cam comes in. It has a more elongated shape and therefore when it is pressed all three rocker arms, and hence the valves, drop much lower and remain open for a longer period of time. In this case, the engine can breathe more freely, develop and maintain high torque, and have good power.
SOHC VTEC
After the success of the DOHC VTEC system, Honda approached the development and use of its innovation with even more zeal. VTEC engines proved to be reliable and economical, providing a viable alternative to larger displacements or the use of turbines. That’s why the SOHC VTEC system was introduced somewhat later. Like its “counterpart” DOHC, the novelty was also intended to optimize engine performance in different modes. But due to its simple design and more modest power output, SOHC VTEC engines were produced in smaller volumes. One of the first engines to use this simplified system was the upgraded D15B, which produced 130 horsepower with a displacement of 1.5 liters. This engine was installed on Honda Civic in 1991.
The SOHC motor has a single camshaft for the whole cylinder block. Therefore, the intake and exhaust cams are located on the same axis. However, there are also clusters of three, each with one special center cam. The simplicity of the design is that only the intake valves can be operated in the two modes – for low and for high rpm. An intermediate mechanism with an extra cam and rocker arm also, as with the DOHC VTEC, intercepts the opening and closing of the intake valves, while the exhaust valves always operate in constant mode.
One might get the impression that SOHC VTEC is somehow worse than DOHC VTEC. However, this is not the case: this system has a number of advantages, among them the simplicity of design, compactness of the engine due to its small width, and lower weight. In addition, SOHC VTEC can quite easily be used on previous generation engines, thereby upgrading them. As a result, SOHC VTEC powertrains achieve the same results, albeit less striking and surprising.
SOHC VTEC-E
If the purpose of the VTEC systems described above is to combine maximum power at the upper rpm and fairly confident but economical operation at the lower rpm, VTEC-E is designed to help the engine achieve maximum economy.
But before we consider another invention of Honda, it is necessary to deal with the theory. It is known that the fuel is premixed with air and then ignited in the cylinders (there is another option – direct injection when the air and fuel enter the cylinders separately). The power of the engine is also affected by how homogeneous the mixture is. The fact is that at low speeds, the low flow rate at intake prevents the mixing of fuel and air. As a result, the engine may run rough at idle. To prevent this, a fuel-enriched mixture is fed into the cylinders, affecting fuel economy. VTEC-E is able to ensure that the engine runs confidently at low RPM with a leaner mixture of fuel. The economy is also substantial. Unlike other gears, VTEC-E has no additional cams. Since this technology aims to reduce fuel consumption at low rpm, it affects the action of the intake valves. VTEC-E is only used in SOHC engines (single camshaft) with four valves per cylinder because of its “propensity” for low fuel consumption.
Unlike other VTEC engines, where the cams have roughly the same profile, powertrains with VTEC-E use two configurations. Thus, the intake valves are driven by different shaped cams. One has a traditional cam profile, while the other is almost round – slightly oval. Therefore, one of the valves lowers normally, while the other is barely open. The combustible mixture passes through the normal valve easily, and through the ajar valve very sparingly. Because of the asymmetry of the incoming mixture flows, bizarre swirls occur in the cylinder where air and fuel mix properly. As a result, the engine can run on a lean mixture. As revs increase, the fuel concentration increases, but the mode where only one valve actually operates becomes a nuisance. Therefore, at approximately 2500 rpm the rocker arm closes and is actuated by the normal cam. Closing occurs in exactly the same way as in other VTEC systems.
The VTEC-E system is often undeservedly considered an invention aimed solely at the economy. Nevertheless, compared to simple engines, units with this mechanism are not only more economical but also more powerful. The first mode, in which one valve works, is responsible for the economy, and the “purebred” VTEC, which implies the wide opening of the intake valves, is responsible for the power indicators. If you compare two similar motors, one of which is equipped with a VTEC-E mechanism, the simple unit will be 6-9% weaker and more voracious.
Three-mode SOHC VTEC
This mechanism is a combination of the SOHC VTEC and SOHC VTEC-E systems. Unlike all the systems described above, this one has not two modes of operation, but three. In the low rpm zone, the system provides lean-mix economy operation (like VTEC-E). In this case, only one of the intake valves is used. At mid-range RPM the second valve is engaged, but the valve timing and lift are not changed. The engine in this case realizes high torque. In high rpm mode, both valves are controlled by a single center cam, which is responsible for removing maximum power from the engine. This system is quite versatile. For example, a 1.5-liter engine with such a timing mechanism exhibits a good specific power: 86 hp per 1 liter of displacement. At the same time, if the engine is working in the first, economical 12-valve mode, the consumption when driving at a constant speed of 60 km / h in a Honda Civic is about 3.5 liters per 100 kilometers.
i-VTEC
The “i” in the name means intelligent, that is, “smart”. The former VTEC versions are able to adjust the degree of valve opening only in 2-3 modes. The design of the new i-VTEC variable valve train assumes the use of an additional VTC (Variable Timing Control) system in addition to the main VTEC system, continuously adjusting the timing of the intake valve opening. The opening of the intake valves is set depending on engine load and is regulated by changing the angle of the inlet camshaft relative to the exhaust camshaft. For engines with i-VTEC, the camshaft is attached to the drive pulley via a special pinion nut that can “turn” it up to an angle of 600.
The use of the VTC system, along with VTEC, allows more efficient filling of the engine cylinders with the fuel-air mixture, as well as improving the completeness of its combustion. The use of i-VTEC allows you to achieve the same level of acceleration as a 2-liter engine, with even better fuel economy than a 1.6-liter engine.
The VTEC timing family is nothing magical, but it has an amazing effect. Honda’s engines know just how to adjust to the load, providing amazing power at a modest displacement. And at the same time at idle and low-speed Japanese motors amaze with the outstanding economy. It is quite possible that the next stage in the development of VTEC systems will be a mechanism with separate solenoids for each valve, which will allow regulating the opening of the valves with surgical precision.