There are plenty of websites which say what the CVT transmission is, but very few websites give a detailed review of Nissan’s implementation of CVT in their vehicles. Nissan uses CVT in several vehicle models, but this review will focus around the Nissan Sentra SL 2.0, 2008. A few months ago, I moved and had no choice but to purchase a car. I decided to get the Nissan Sentra because of its introductory price and high fuel economy. Before I get into the review, allow me to explain just what normal and CVT transmissions are.
Normal transmissions have a specific number of gears, anywhere from 4 to 6 gears, usually. While this is the most widely used type of transmission, there are several problems with it. One major problem is that car engines run most efficiently at specific RPM and torque curves. Normal transmissions are designed with gear ratios to match for optimality as close as possible, but the engine is rarely running with an optimal gear ratio in your normal day to day driving scenarios. Normal transmissions also have one more notable disadvantage. When switching from one gear to another, the car looses torque momentarily. Switching gears is one of the most noticeable and undesirable features of normal vehicle transmissions.
CVT = Continuously Variable Transmission
CVT transmissions are different. As the abbreviation may indicate, a CVT transmission has a nearly infinite number of gear ratios. The transmission can switch between these gear ratios seamlessly. In theory, this should completely eliminate the problem of shifting gears. Shifting gears is an unpleasant feeling and wastes a surprising amount of gas. Therefore, in theory, the CVT should be able to save you some gas money. The other major advantage to CVT transmissions is that they can keep the effective gear ratio optimal for the engine’s current RPM and desired torque. This is another major feature which can save you some serious gas money. Often, you will find yourself driving, and you’ll notice that sometimes you push the gas pedal down to get more torque and the engine stays at the same RPM! This is because the CVT transmission is simply changing the effective gear ratio in order to have the most fuel efficient operation.
Nissan Sentra’s implementation of the CVT transmission
Now for the actual review! First, we can take care of the positives. One feature that is cool about the Nissan Sentra is when the car is idling. All cars have an idle speed set so that the car slowly moves forward when you release the brake pedal. This idle speed is the same whether or not you are creeping through a parking lot, or stopped at a stopping light. Nissan Sentra’s transmission takes a completely different, gas saving approach. When you stop the vehicle for more than a second, the CVT adjusts itself to allow the car to use less gas when idling. This means that whenever you are stopped at a stop light, your car will use much less gas when idling, which will save you quite a bit of money. When you release the brake pedal, the CVT adjusts to give the engine enough torque to slowly move the car forward just like a normal car.
The other great thing about Nissan’s CVT is that it really does keep your engine at the optimum gear ratio while you’re driving steadily. This is perhaps the biggest fuel saver of all, especially when you’re constantly in motion, and not in stop and go traffic.
Disadvantages of Nissan’s CVT
Even though CVTs have been used for years, Nissan still manages to really screw it up. First things first, let me explain a little bit about who I am. I’m an electrical engineer, and I’ve taken many classes pertaining to embedded systems and signals and systems. The gear ratios for the CVT are controlled purely through software which runs on an embedded processor in the car. Unfortunately for Nissan, they put in a great transmission, but the embedded software is seriously lacking. The Nissan Sentra could’ve been a great vehicle if they had just spent more time writing decent control software for the transmission. The following problems are caused completely or almost completely by software problems in the Nissan Sentra.
Problem 1 - The CVT simulates geared transmissions
The software controlling the CVT transmission is designed to simulate a geared transmission. That’s right, you read that part correctly. The biggest advantage of the CVT is that it can theoretically keep the engine at the most fuel efficient gear ratio. Well, Nissan throws that idea out the window when you’re starting the car from a complete stop. Suppose you are at a stop light, and you are at a complete stop. The light turns green and you accelerate to 45 miles an hour. During this acceleration, the engine will actually rev up, then simulate a gear change and rev back down. This wastes gas, and it wastes your money. Although this transition between two imaginary gears is much smoother than normal transmissions, you still lose torque and fuel economy. The only reason this happens is because some project manager at Nissan decided drivers would be more comfortable with a familiar gear transition while accelerating. The saddest part of this problem is that it’s all because the software is poorly implemented. Why would someone buy a CVT transmission just so it can simulate a normal transmission?
Problem 2 - The CVT intentionally slows your car down when coasting
Another serious problem is how engine braking is handled. Usually, when you are driving and release the gas pedal, your car will slow down. This is partly due to tire friction, wind resistance, and the engine is also slowing down the car through the transmission. Unfortunately, our friends at Nissan decided to make the CVT simulate the engine braking of normal transmissions. Of course, this is implemented in software only. What does this mean for you? It means that when you release the gas pedal and start to coast, the CVT actually slows down the car quite fast. Obviously, if you coast a lot in order to save gas, you can expect this ‘feature’ to waste a lot of money. The best way to get around this is by pressing the gas pedal extremely gently. Very little gas will go to the engine, and the CVT will no longer engage in engine braking. Honestly, I’m not sure what Nissan’s engineers were thinking when they decided to implement this. I have come across a forum where a person had successfully reprogrammed his CVT transmission to disable engine braking completely. But unless you’re a skilled mechanic with very expensive tools, you are not likely to be successful in disabling the CVT engine braking.
Problem 3 - The CVT can’t change effective gear ratios fast enough
If you’re following someone too closely, you may have to engage the gas pedal, then the brake pedal, and then the gas pedal again. Unfortunately, there is a physical limitation to how fast Nissan’s CVT can change effective gear ratios. In the scenario described above, you can expect your CVT to jerk the car severely. This is actually a physical problem with the CVT design, not the software. The best way to avoid this is by changing your driving habits. If you apply the gas pedal, then the brake, when you release the brake, give yourself about one and a half seconds of time until you step on the gas pedal again. This will avoid the jerk.
Problem 4 - Constant droning noise
Noise is something none of us like to hear when driving our cars. Now, I’ve read many sites that overstate the noise the transmission makes. Yes, it is audible when you are accelerating, but I’m rarely bothered by it. Some people online seem to find the noise annoying, so be sure to pay attention during your test drive to see whether or not you are bothered by a low, constant droning sound.
Well, there you have it. Hopefully, this article has given you some insight as to what the Nissan Sentra’s CVT transmission is like, and some serious downfalls to the transmission. Overall, it’s possible to get very good fuel economy partly due to the transmission design. For example, when I’m going 55 miles per hour on a highway with cruise control on, I can expect to get 45 miles per gallon. When I go 60 miles per hour on the highway, I can expect to get 43 miles per gallon, which is great. Nissan’s CVT transmission is not perfect, and many of the flaws can be blamed on software, not hardware. Hopefully Nissan’s next release of the Sentra will include a better transmission with better control software.
If you enjoyed this article and would like to view more gas saving tips and articles please check out these articles, or any posts listed under the ‘saving gas’ category. Remember to check back often for updates and more articles on how to save money.
Posted under Saving Gas, Saving Money, Shopping
This post was written by admin on September 10, 2008