The Efficiency of Transmissions: Understanding the Ideal Driving Scenario

So as you step out of that situation where you're driving at high RPM and low load, that's when your transmission is most efficient. This is because efficiency trends towards high load, low RPM scenarios. So let's say you were in third gear and you're at a fairly high RPM and a fairly low load, well perhaps you could be in fifth gear at a higher load. The RPM will decrease but the load will increase and that's when you'll be producing the exact same amount of power but using less fuel, which is ideal.

This highlights the importance of having multiple gear ratios to place yourself in this optimal scenario. More gears mean more opportunities to put yourself in an efficient position. Another reason why CVTs are common today because they can put you in that really efficient spot. But if you don't want to use a CVT, having more gear ratios gives you the ability to put yourself in that efficient area and still get good fuel economy.

Transmission Gear Range

Transmissions are choosing a gear which is essentially a compromise between acceleration, top speed, and efficiency. As you get into the taller gears, you have higher top speeds and generally will be at lower RPM, which means you will have greater efficiency. But you sacrifice acceleration. The lower gears with higher gear ratios allow you to accelerate quickly.

For example, the Lexus RCF has an 8-speed automatic transmission using basically the exact same engine as the LC 500, but the LC 500 has a 10 speed automatic transmission. So the LC 500's first gear is more aggressive than the RCF's and the LC 500's 9th and 10th gear are taller than the RCF's 8th gear. This gives it a wider overall range where it has more to choose from and more choices for do I want acceleration? Or do I want top speed? Do I want greater efficiency on the highway?

Having a wide range of gear ratios can be beneficial, but if the spacing between those two gears is really wide, you sacrifice acceleration and efficiency because you're going to have this big area of the curve that drops and you're not going to be able to place yourself in that efficient range. You want to have small spacing between those gear ratios to allow for you to stay in that ideal acceleration and to allow for you to be placed in that ideal efficiency scenario, where the engine produces power than most efficiently.

The Importance of Proper Transmission Design

Just sharing my final thoughts on this transmission, driving it I was pretty impressed. Overall, it's pretty good. And so, you know, if you're driving it hard both up shifts and down shifts are very quick, which I quite like. When you're not driving it that hard, down shifts are a little bit rough. It doesn't seem to search all that much for gears, which I was surprised by, it does seem to have this sort of play, it felt like, within the drivetrain where there kind of is a jerky experience.

And so I thought it was actually downshifting, and then I looked at the readout and it wasn't. So overall, transmission is good, I think it's more meant to be driven hard than it is to just kind of cruise around. But the car itself, you know, comfortable and fast.

Efficiency on the Road

One of the things that's kind of impressive, I went on about a 400 mile road trip driving about 75 miles per hour on the highway, and I got 29 miles per gallon in this thing. So a five liter V8 getting 29 miles per gallon is pretty good. And then also at 85 miles per hour the engine is spinning at just 1750 RPM somewhere around 1750 RPM, pretty impressive how low you can get those RPMs on the highway.

This shows that the transmission can switch over to the Atkinson cycle, which helps with efficiency when you're not flooring it. So when you know, you're getting on it, then it's going to use the auto cycle and then it'll use valve timing to use the Atkinson cycle when you're on lower load scenarios. Just cruising on the highway.

A Final Word

If you guys have any questions or comments, of course, feel free to leave those below. Thanks for watching.

"WEBVTTKind: captionsLanguage: enHello everyone and welcome in this video, we are talking about 10 speed automatic transmissionswhy? Well recently I have been driving the Lexus LC 500 which Lexus claims is the firstluxury vehicle to use a 10 speed automaticCoolNow, do we really need this many gears?Well, we're gonna be talking about three reasons why it could actually be pretty logical to use a 10 speed transmissionSo we're gonna be talking about acceleration, efficiency, and gear rangeand so first starting off with accelerationWe've got this little graph here and essentially what it isWhat we're seeing is wheel torque on the leftWhich is basically just the force that is pushing the car forward, the force that those wheels are pushing forwardAnd then vehicle speed on the bottom. And so this blue curve here. This is the maximum possibleAnd so what this is showing is you knowAs your vehicle speed is lowerThe maximum wheel torque is higherThen if your vehicle speed is higher, if your vehicle speed gets higher the maximum forceYou can accelerate your car with decreases. Now why is that? Well, that's because you're using lower gear ratiosSo here let's say you're in first gear and you can accelerate quite hard.first gear can only take you up to say maybe 30-40 miles per hourAnd then you've got to switch gears to a lower gear ratio to reach higher speedsAnd so this right here shows you what is the ideal curve that you could remain onto have the highest acceleration possible.And how that is achieved is by putting the engine at peak horsepowerSo let's say right hereWe are at 45 miles an hour and there are some gear ratio that allows us to be at 7100 rpmThat's when the Lexus LC 500 produces peak horsepowerIf we have that gear ratioThen we have the peak acceleration for that vehicle speed now that actually teaches us whyCVTs are ideal because they can hold the engine any rpm and then change the gear ratio as neededSo a CVT could just hold the engine at 7100 rpm and then accelerate from, you knowThe low gear ratio all the way through the high gear ratio and remain on this perfect acceleration curve.But people don't want to spend $100,000 on a sports car and have a CVT.They enjoy the gear shifts, they enjoy that transmission shifting gearsAnd, you know, the associated noises and feel that comes with itAnd so you don't tend to see CVTs in sports cars. So instead you have gear ratios.Well gears sacrifice performance.So, if you're looking at this line, you know, let's say you're in first gear. And here's your torque curve hereSo it allows you to get close to and then reach that peakYou know, acceleration possible as you top out in that gear and then it starts to taper offand then you have to switch gears, so when you switch gears you drop in accelerationbecause your gear ratio is dropping and then you get on to gear 2and so you have this area right here where you're losing accelerationbecause if you had a bunch of gears you could remain on this lineif you had infinite gears you could remain on that lineand so by adding in another gear, like having 10 gear ratiosYou can keep yourself closer to that ideal curve and still have gearstill have the joy of driving that is associated with transmissions.Now a couple of things to point out here, some people may say that CBTs can't handle the torquecan't handle the horsepower for these scenarios, these larger enginesI say that's nonsense if Williams could do it in the early 1990s in Formula 1Then surely with today's technology and today's brilliant engineersWe are capable of high torque CVTs, why it's not done is because people aren't going to buy it.I think it'd be super cool if some supercar company just kind of trolled the world andThrew a CVT in the supercar set a bunch of lap records. I think that would be neat to seebut they probably wouldn't make any money and so they're probably not going to do it and then alsoYou know adding gears there is a downside of course to doing that because it takes time to shift gears nowI don't think that's that big of a deal because first of all shifting gears is kind of fun and second of allyou know, if it means you're gettingMore g-forces, if it means the g-force is the force pushing you forward is higher then that's you knowMore of the joy of driving. So maybe overall it's slower because your shift times are making you slowerBut if the force pressing you back in your seat is always higher except for when you're shifting gears then to meI'd rather have that in a slower car because it's going to feel more fun so I kind of like the idea of that.On top of this Lexus says this automatic transmission is capable of shifting in just 0.12 secondsOkay. So now let's talk about efficiency, so here I have a plot of brake specific fuel consumptionBrake specific fuel consumption is how much fuel is required to make a specific amount of power.And so here on the Left we have load. This is essentially how much throttle are you giving the car?And then here on the bottom we have RPM and so the theme, the color scheme here, is basically a ROYGBIVWhere we've got red being the most efficient, orange is also efficient; but less efficient than redand then it starts to decrease out as you get to purple which here represents the least efficient areaSo you want to be in this low RPM, high load scenario to have maximum efficiencyAnd so as you step out of that as you get into low load, high RPM scenarios, that's going to be your least efficient.So efficiency trends towards high load, low RPM scenarios. So let's say you were in third gear andYou're at a fairly high rpm and a fairly low loadWell, perhaps you could be in fifth gear at a higher loadSo the RPM is going to decrease but the load is going to increase and that's going to put you, you knowYou're going to be producing the exact same amount of powerBut you're doing so using less fuel, so that's ideal.So the more gears that you have the more opportunityYou have to place yourself in this sweet spotAgain, another reason why CVTs are common today because they can put you in that really efficient spotBut if you don't want to use a CVT, having more gear ratiosGives you the ability to put yourself in that efficient area and still get good fuel economy.Next. Let's talk about gear rangeSo transmissions are choosing a gear which is essentially a compromise between acceleration, and top speed, and efficiencySo as you get into the taller gears you have higher top speeds and you generally will be at lower RPMWhich means you will have greater efficiencyBut you sacrifice acceleration, the lower gears with higher gear ratios, allow you to accelerate quicklyAnd so, for example, we the Lexus RCF which has an 8-speed automatic transmissionUsing basically the exact same engine as the LC 500, but the LC 500 has a 10 speed automatic transmission.So the LC 500's first gear is more aggressive than the RCF's and the LC 500's 9th and 10th gear are taller than the RCF's 8th gear.And so it has a wider overall range where it has more to choose from and has more choices for do I want acceleration?Or do I want top speed? Do I want greater efficiency on the highway? And so you're thinking to yourselfOkay, well, all you need is just two gears to have a wide range. You can put them wherever you want.And that's absolutely true, but if the spacing between those 2 gears is really wideYou sacrifice acceleration and you sacrifice efficiency because you're gonna have this big area of the curve that dropsand you're not going to be able to place yourself in that efficient rangeso you want to have small spacing between those gear ratios to allow for you to stay in that ideal accelerationand to allow for you to be placed in that ideal efficiency scenario, where the engine produces power than most efficiently.So that 10 speed transmission offers a wide range of gear ratios, without sacrificing acceleration, or efficiency.So just sharing my final thoughts on this transmission, driving it I was pretty impressed.Overall, It's pretty good. And so, you know, if you're driving it hardBoth up shifts and down shifts are very quick, which I quite like.When you're not driving it that hard, down shifts are a little bit rough.And it doesn't seem to search all that much for gears, which I was surprised by, it does seem to, you knowIf you don't give it too much throttle, if you just give it a little bit of tip in on the throttleThen it won't automatically just, you know, shift to a lower gear ratioBut it does have this sort of play, it felt like, within the drivetrain, where there kind of is a jerky experienceAnd so I thought it was actually downshifting, and then I looked at the readout and it wasn't.So overall, transmission is good, I think it's more meant to be driven hard than it is to just kind of cruise aroundBut the car itself, you know, comfortable and fastSo one of the things that's kind of impressive, I went on about a 400 mile road tripdriving about 75 miles per hour on the highway, and I got 29 miles per gallon in this thingSo a five liter V8 getting 29 miles per gallonPretty good, and then also at 85 miles per hour the engine is spinning at just 1750 RPMsomewhere around 1750 RPM, pretty impressive how low you can get those RPMs on the highwaySo I like that and it does switch over to the Atkinson cycle, which helps with efficiency when you're not flooring itSo when, you know, you're getting on it, then it's going to use the auto cycle and then it'll use valve timing toUse the Atkinson cycle when you're on lower load scenarios. Just cruising on the highway.So if you guys have any questions or comments, of course, feel free to leave those belowThanks for watching.\n"