Feedman
06-10-2004, 12:06 AM
VTEC is an acronym for Variable valve Timing and lift Electronic Control. It is a mechanism for optimizing air/fuel mixture flow through the engine.
An internal combustion engine converts the chemical energy stored in fuel into thermal energy. The increased thermal energy within a cylinder causes the pressure to build. This pressure acts on the pistons and the result is a mechanical force rotating the crankshaft. This mechanical force is measured as crank torque. The ability for the engine to sustain a certain level of crank torque at a certain RPM is measured as Power. Power is the rate at which the engine can do work. This conversion process is not 100% efficient. In fact, only about 30% of the energy stored in the fuel is actually converted into mechanical energy.
Physics says that for a given efficiency level, a higher rate of fuel consumption is needed for the engine to generate power. So it becomes obvious that if you want more power, you need to increase the rate of fuel combustion. One way to achive this goal is to have a bigger engine. A bigger engine with larger cylinders will be able to combust more fuel per rotation than a smaller engine. Another method is to pre-presurize the fuel/air mixture and cram it into an existing engine size. Thus even though the cylinder size stays the same, more fuel is combusted per rotation. This second method is referred to as forced induction(boost).
Honda chose to explore another method: keep the engine size the same, but turn the engine faster to consume more fuel. Here is an analogy: You want to move foam peanuts from one bucket to another with a cup. 1.) You can increase the size of your cup. 2.) Compress/cram as much peanuts as possible into the cup each time 3.) You can just move the cup faster. All three methods moves more peanuts. Honda uses the last method. And again, more fuel combusted equals more power generated by the engine.
As the engine speed is increased, more air/fuel mixture needs to be "inhaled" and "exhaled" by the engine. Thus to sustain high engine speeds, the intake and exhaust valves needs to open nice and wide. Otherwise you have what is akin to athsma: can't get enough air/fuel due to restrictions.
If high speed operation is all we have to worry about, Honda wouldn't need to implement VTEC. Indeed, race engines that operate mostly at high rpms do not utilize any mechanism like VTEC. But street cars used for daily driving spend most of their time with the engine at low RPMs. Valves that open wide for high RPM operation contributes to rough operation and poor fuel economy at low RPMs. These undesirable traits are directly against Honda's design goals.
The solution that Honda came up with is the VTEC mechanism: open the valves nice and wide at high RPMs, but open them not as much at low RPMs. So now you have a engine with smooth operation at low RPMs, and high power output at high RPMs.
And that is basically what VTEC is. It's nothing magical. The idea has been around for a long time. Honda's VTEC is just a very simple, elegant and efficient implementation that is extremely effective at achiving its design goal. Honda automobiles are the first among modern automobiles to utilize this mechanism in such a large scale of distribution.
Pros
The main benefit of VTEC is that the resulting engine is very versatile. The torque curve is very flat: among the flatest of all the engines on the market. Thus where other engines are running out of breath, a VTEC engine maintains a nice and steady output of torque, making the whole RPM range usable for acceleration. So when you are just driving around at a reasonable pace, the car is very smooth and fuel consumption is similar to other engines of the same displacement. When you need more power for passing, all you have to do is down shift and take advantage of the extra power available at the higher RPMs. So you get the smoothness and fuel efficiency of a small economical engine when you drive a low RPMs, and the power output of a much larger engine at high RPMs.
Due to the greater range of usable RPMs, shorter gears can be used. Thus for any given speed or engine RPM, a VTEC engine will allow for a larger ratio multiplier, resulting in more wheel torque. Thus the benefit of the VTEC technology in terms of acceleration improvement also affects low RPM operation.
Since VTEC creates more power without increasing displacement, the engine is likely to be smaller and lighter. For example: 1.5L 1.6L 1.8L 2.0L 2.2L
Cons
A vehicle achives its greatest acceleration by keeping the engine RPM as close to the HP peak as possible. And for DOHC VTEC engines, this means keeping the needle at some rather lofty RPMs, and more frequent shifts to keep the RPMs up. To some people, this is a desirable trait: lots of driver involvement in the process of extracting excellent performance. To others, especially those accustomed to the Kansas-flat HP curves of muscle cars, the high RPM and frequent shifts become bothersome.
For a good launch off the line, such as at the start of a drag race, a certain amount of tire spin is desired. Muscle cars have torque peaks at low RPMs, and then taper off as the RPM builds. This is perfect for drag racing as the initial torque peak generates the desired tire spin, and then the lower torque at higher RPMs allow the tire to find and maintain grip. But DOHC VTEC's torque curve is very flat, so the initial tire slip is much harder to generate. And once the tire looses traction, the flat torque curve makes it hard for the spinning wheels to find traction. So to properly launch a DOHC VTEC car, the driver must slip the clutch at high RPMs to generate the initial tire spin, and then carefully modulate the clutch and gas to regain drive wheel traction while maintaining maximum acceleration.
Even though Honda's VTEC engines has lived up to the legendary reliability of Honda products, the fact remains that having the VTEC mechanism adds complexity and cost.
Vtec Misconceptions
Honda's use of the VTEC technology has lead to some pretty impressive achivements. This results in three camps of people: those that think VTEC is nothing but hype, those that know what VTEC is in terms of its benefits and limitations, and finally, those that think VTEC is the best thing to happen to automobiles since round wheels. Inevitably, misconceptions about VTEC are formed and thrown around. Much of the arguments on such forums as rec.autos.makers.honda is caused by such misconceptions. Here are some rather common ones:
DOHC VTEC engines have low crank torque compared to non VTEC engines of similar power output, and crank torque alone is an important indication of how well an engine will accelerate a car. Therefore the VTEC engine's power rating is not "real".
An engine's crank torque is directly related to how much fuel/air is combusted per engine cycle. For normally aspirated engines, this means that increasing the displacement size will usually result in increased crank torque. For forced induction engines, the effective displacement is larger than the numerical displacement since the air is pre-compressed before it is forced into the engine. Unlike increased displacement or forced induction, the VTEC system optimizes engine breathing at high RPMs to increase power. Therefore, a VTEC engine's displacement is the smallest of the three methods of increasing power output. And since crank torque is limited by displacement, a VTEC engine's crank torque output is smaller compared to non-VTEC engines of similar power output level.
But this doesn't mean that a VTEC engine's HP is somehow worth less. In fact, Honda automobiles equipped with VTEC engines have performance numbers that agrees with the tried and true power-to-weight-ratio method of estimating acceleration performance. People hold this misconception because they have a fundamental lack of understanding of the relationship between crank torque, horse power, and acceleration. Crank torque by itself is meaningless in determining the engine's ability to accelerate the car. This is because the crank torque is multiplied by the gearing and final drive ratio before it is converted to forward thrust. And physics dictates that an engine putting out 160HP absolutely will provide more forward thrust than a 150HP engine, regardless of what crank torque the two engines have, assuming similar transmission efficiency and optimal gearing for both cars. This is plain high school physics. Unless someone can prove that the laws of thermodynamics and Newtonian physics are false, there is no way around this fact.
There is some significance to the shape of the crank torque curve, however. When drag racing a car, it is desired to have a bit of initial wheel spin, and then have the tire hook up with the ground. A torque curve with a peak early in the RPM range and then tapers off as RPMs rise is well suited to this purpose. This is why big displacement American muscle cars are so good at drag racing. VTEC engines, on the other hand, have very smooth gradually rising torque curves. The initial wheel spin, therefore, is harder to achive. And after the initial wheel spin gets going, the level torque curve means that very precise clutch and gas pedal control is needed to allow the drive wheels to regain traction while maintaining maximum acceleration. This is why VTEC engines are more difficult to launch off the line than large displacement muscle car engines.
Alot of this information has been taken from this webiste http://www.leecao.com/honda/index.html
An internal combustion engine converts the chemical energy stored in fuel into thermal energy. The increased thermal energy within a cylinder causes the pressure to build. This pressure acts on the pistons and the result is a mechanical force rotating the crankshaft. This mechanical force is measured as crank torque. The ability for the engine to sustain a certain level of crank torque at a certain RPM is measured as Power. Power is the rate at which the engine can do work. This conversion process is not 100% efficient. In fact, only about 30% of the energy stored in the fuel is actually converted into mechanical energy.
Physics says that for a given efficiency level, a higher rate of fuel consumption is needed for the engine to generate power. So it becomes obvious that if you want more power, you need to increase the rate of fuel combustion. One way to achive this goal is to have a bigger engine. A bigger engine with larger cylinders will be able to combust more fuel per rotation than a smaller engine. Another method is to pre-presurize the fuel/air mixture and cram it into an existing engine size. Thus even though the cylinder size stays the same, more fuel is combusted per rotation. This second method is referred to as forced induction(boost).
Honda chose to explore another method: keep the engine size the same, but turn the engine faster to consume more fuel. Here is an analogy: You want to move foam peanuts from one bucket to another with a cup. 1.) You can increase the size of your cup. 2.) Compress/cram as much peanuts as possible into the cup each time 3.) You can just move the cup faster. All three methods moves more peanuts. Honda uses the last method. And again, more fuel combusted equals more power generated by the engine.
As the engine speed is increased, more air/fuel mixture needs to be "inhaled" and "exhaled" by the engine. Thus to sustain high engine speeds, the intake and exhaust valves needs to open nice and wide. Otherwise you have what is akin to athsma: can't get enough air/fuel due to restrictions.
If high speed operation is all we have to worry about, Honda wouldn't need to implement VTEC. Indeed, race engines that operate mostly at high rpms do not utilize any mechanism like VTEC. But street cars used for daily driving spend most of their time with the engine at low RPMs. Valves that open wide for high RPM operation contributes to rough operation and poor fuel economy at low RPMs. These undesirable traits are directly against Honda's design goals.
The solution that Honda came up with is the VTEC mechanism: open the valves nice and wide at high RPMs, but open them not as much at low RPMs. So now you have a engine with smooth operation at low RPMs, and high power output at high RPMs.
And that is basically what VTEC is. It's nothing magical. The idea has been around for a long time. Honda's VTEC is just a very simple, elegant and efficient implementation that is extremely effective at achiving its design goal. Honda automobiles are the first among modern automobiles to utilize this mechanism in such a large scale of distribution.
Pros
The main benefit of VTEC is that the resulting engine is very versatile. The torque curve is very flat: among the flatest of all the engines on the market. Thus where other engines are running out of breath, a VTEC engine maintains a nice and steady output of torque, making the whole RPM range usable for acceleration. So when you are just driving around at a reasonable pace, the car is very smooth and fuel consumption is similar to other engines of the same displacement. When you need more power for passing, all you have to do is down shift and take advantage of the extra power available at the higher RPMs. So you get the smoothness and fuel efficiency of a small economical engine when you drive a low RPMs, and the power output of a much larger engine at high RPMs.
Due to the greater range of usable RPMs, shorter gears can be used. Thus for any given speed or engine RPM, a VTEC engine will allow for a larger ratio multiplier, resulting in more wheel torque. Thus the benefit of the VTEC technology in terms of acceleration improvement also affects low RPM operation.
Since VTEC creates more power without increasing displacement, the engine is likely to be smaller and lighter. For example: 1.5L 1.6L 1.8L 2.0L 2.2L
Cons
A vehicle achives its greatest acceleration by keeping the engine RPM as close to the HP peak as possible. And for DOHC VTEC engines, this means keeping the needle at some rather lofty RPMs, and more frequent shifts to keep the RPMs up. To some people, this is a desirable trait: lots of driver involvement in the process of extracting excellent performance. To others, especially those accustomed to the Kansas-flat HP curves of muscle cars, the high RPM and frequent shifts become bothersome.
For a good launch off the line, such as at the start of a drag race, a certain amount of tire spin is desired. Muscle cars have torque peaks at low RPMs, and then taper off as the RPM builds. This is perfect for drag racing as the initial torque peak generates the desired tire spin, and then the lower torque at higher RPMs allow the tire to find and maintain grip. But DOHC VTEC's torque curve is very flat, so the initial tire slip is much harder to generate. And once the tire looses traction, the flat torque curve makes it hard for the spinning wheels to find traction. So to properly launch a DOHC VTEC car, the driver must slip the clutch at high RPMs to generate the initial tire spin, and then carefully modulate the clutch and gas to regain drive wheel traction while maintaining maximum acceleration.
Even though Honda's VTEC engines has lived up to the legendary reliability of Honda products, the fact remains that having the VTEC mechanism adds complexity and cost.
Vtec Misconceptions
Honda's use of the VTEC technology has lead to some pretty impressive achivements. This results in three camps of people: those that think VTEC is nothing but hype, those that know what VTEC is in terms of its benefits and limitations, and finally, those that think VTEC is the best thing to happen to automobiles since round wheels. Inevitably, misconceptions about VTEC are formed and thrown around. Much of the arguments on such forums as rec.autos.makers.honda is caused by such misconceptions. Here are some rather common ones:
DOHC VTEC engines have low crank torque compared to non VTEC engines of similar power output, and crank torque alone is an important indication of how well an engine will accelerate a car. Therefore the VTEC engine's power rating is not "real".
An engine's crank torque is directly related to how much fuel/air is combusted per engine cycle. For normally aspirated engines, this means that increasing the displacement size will usually result in increased crank torque. For forced induction engines, the effective displacement is larger than the numerical displacement since the air is pre-compressed before it is forced into the engine. Unlike increased displacement or forced induction, the VTEC system optimizes engine breathing at high RPMs to increase power. Therefore, a VTEC engine's displacement is the smallest of the three methods of increasing power output. And since crank torque is limited by displacement, a VTEC engine's crank torque output is smaller compared to non-VTEC engines of similar power output level.
But this doesn't mean that a VTEC engine's HP is somehow worth less. In fact, Honda automobiles equipped with VTEC engines have performance numbers that agrees with the tried and true power-to-weight-ratio method of estimating acceleration performance. People hold this misconception because they have a fundamental lack of understanding of the relationship between crank torque, horse power, and acceleration. Crank torque by itself is meaningless in determining the engine's ability to accelerate the car. This is because the crank torque is multiplied by the gearing and final drive ratio before it is converted to forward thrust. And physics dictates that an engine putting out 160HP absolutely will provide more forward thrust than a 150HP engine, regardless of what crank torque the two engines have, assuming similar transmission efficiency and optimal gearing for both cars. This is plain high school physics. Unless someone can prove that the laws of thermodynamics and Newtonian physics are false, there is no way around this fact.
There is some significance to the shape of the crank torque curve, however. When drag racing a car, it is desired to have a bit of initial wheel spin, and then have the tire hook up with the ground. A torque curve with a peak early in the RPM range and then tapers off as RPMs rise is well suited to this purpose. This is why big displacement American muscle cars are so good at drag racing. VTEC engines, on the other hand, have very smooth gradually rising torque curves. The initial wheel spin, therefore, is harder to achive. And after the initial wheel spin gets going, the level torque curve means that very precise clutch and gas pedal control is needed to allow the drive wheels to regain traction while maintaining maximum acceleration. This is why VTEC engines are more difficult to launch off the line than large displacement muscle car engines.
Alot of this information has been taken from this webiste http://www.leecao.com/honda/index.html