Surprising fact # 1: Car performance depends on horsepower alone. Torque and RPM are little more than ingredients and are suprisingly irrelevant for final performance. (Assuming the usage of an ideal transmission for the engine and vehicle in question) B… but… my big displacement torque 8 cylinder diesel with its 4-digit-Nm-value feels unstoppable?!? Search your feelings and I promise you, torque is not what you feel. Horsepower = RPM multiplied by Nm – yes Newton meters... leave me alone with your quirky american measurements….. we are already talking about horses… that has to be enough accommodation for you wild west descendants. You probably already knew that and understood that those values are related to each other but what do they actually mean? Well, here is an unrealistic but hopefully easy to understand way of illustration: All numbers were choosen for illustration purposes only and not for realism. Imagine 2 construction workers at a construction site: Mr. Petrol and Mr. Diesel Their boss orders each of them to move 1000 kg cement bags one level upstairs. They have one hour of time to do so. Mr. Petrol, a lanky runner type of guy, grabs the first 25 kg cement bag and carries it one level up. He needs 90 seconds per bag and as such, after running 40 times, has transported: 40 X 25kg = 1000 kg in 90 seconds X 40 = 3600 seconds = 60 minutes = 1 hour. Mr. Diesel, a bear of a man, grabs two 25kg = 50kg cement bags before he – somewhat slowly and cumbersome – carries them one level up. He needs 180 seconds for a single round with 2 bags. He has to walk 20 times in total: 20 X 50kg = 1000 kg in 180 seconds X 20 = 3600 seconds = 60 minutes = 1 hour. The hour has passed and both construction workers finished their job just in time. Question….. which construction worker did more work? Who will get more praise from his boss? Anwer: Both of them did the same amount of work…. they both moved 1000 kg of cement one level upstairs in 1 hour of time. They get the same praise from their boss because they did exactly the same amount of work. In car terms we would say… they both have the SAME HORSEPOWER as they can do the SAME amount of work in the SAME amount of time. Mr. Petrol carries 25 kg per run but runs very fast. Mr. Diesel carries 50 kg per run but walks much slower. The END RESULT… it identical. They merely achieve the same work/horsepower by different means. Mr. Petrol has low carry capacity/torque and as such needs to run quite often/needs high RPM to do the work on time. Mr. Diesel has high carry capacity/torque but can only walk much slower/needs lower RPM to do the work on time. If you want more work done/more horsepower you have to increase either the carry capacity/torque or run more often in the same amount of time/increase RPM: You can generate 100 Horsepower: in Mr. Diesels way: 236 Nm @ 3000 RPM = 100 HP in Mr. Petrols way: 118 Nm @ 6000 RPM =100 HP in Mr. Electrics way: 59 Nm @ 12000 RPM = 100 HP Have only half the torque? You need to double the revs. Have twice the torque? You can create the same amount of work/horsepower at half the RPM. Example: Maintaining a speed requires the same amount of horsepower no matter the power source (ignoring mass, aerodynamics and rolling resistance which are slightly different here) @100 km/h: Diesel: 1750 RPM - 116Nm - 28,45 HP Wheel Torque: 218 Nm - more than twice the value of the flywheel, increased by transmission gear in the drivetrain. Gasoline: 2253 RPM - 85 Nm - 26,94 HP Wheel Torque: 200 Nm - basically identical to the diesels one. The higher engine RPM and a different gear ratio in the drivetrain increase the inferior engine torque to identical levels. Electric - 6633 RPM - 31 Nm - 28,48 HP Wheel Torque: 236Nm - notice the lower drivetrain losses of the simple single speed electric drivetrain transmission. We always create arround 26 - 28 HP with different RPMs and torque values. Quod erat demonstrandum. @150 km/h - same as above just with higher numbers: Diesel 2614 RPM - 169 Nm - 61,98 HP - a bad moment to take a screenshot, I hit the moment where the cruise control increased engine load and as such overshoots the "50 something" HP of the gasoline and electric car by almost 10 HP in this very moment. Gasoline 3357 RPM - 112 Nm - 52,90 HP Electric 9839 RPM - 37 Nm - 50,52 PS - less drivetrain losses mean more of the rated HP can be put into actual movement, good for electricity economy and range. Torque says very little about a cars performance. As you can see, a proper transmission can multiply torque to the desired level at the wheels. Power is the actual work done and as such is almost solely responsible for how fast your car can accelerate and what top speed it can reach. Torque is little more than an indicator at which RPM one can expect a certain power output. Your 8 cylinder diesel does not feel powerful because of its torque... it feels powerful because this torque grant it significant power at very low RPMs. And much smaller gasoline engine could give you precisely the same power, but it would require a very different gear ratio and ultimately much higher RPM. Since in regular driving most people try to avoid high RPM, lower torque cars often feel underwhelming in comparsion to their rated power. Diesel 110 HP @ 3660 RPM - that is not low RPM anymore but still quite reasonable. Gasoline 110 HP @ 5034 RPM - we are getting a bit loud and strained, arent we? This is to loud. Lets reduce RPM to the diesels level: Much better. But wait... what happened to my power? Only 93 HP instead of 110 HP. 17 HP less for the same RPM. --> The Gasoline car will have a reduced performance of 17 HP in comparsion to the diesel car at the same RPM. This is why it feels - and is - weaker. Not because of lower torque but because this lower torque results in lower HORSEPOWER at the same RPM compared to the diesel. We are Mr. Petrol but we have decided to not run faster than Mr. Diesel.... of course our total output is lower. Super/Turbocharged cars in general and turbodiesels in particular feel very powerful because they deliver a larger percentage of their rated power at "reasonable RPM ranges". Gasoline cars may have the same or even superior power output but the "screaming revs" required to access that power make most people shy away from them, resulting in a lower amount of horsepowers on the road compared to turbodiesels of similar rated peak power. Real life example: Ford Focus Mk 1: 1.6L Gasoline: 100 HP @ 6000 RPM 1.6L Turbodiesel: 90 HP @ 4000 RPM Yes, the gasoline car has 10 HP more power. But it requires 5500 RPM to reach 90 HP. 1500 RPM more than the turbodiesel one. Few people use those revs and as a result the turbodiesel feels much more powerful between 1000 - 4000 RPM. Only between 5500 - 6000 RPM - does the gasoline car actually has superior horsepower. You need to rev to 5500 RPM just to have the same performance in your gasoline Focus than in your Turbodiesel one. If you want to be faster, you have to rev even higher all the way to 6000.... only then you can actually beat the turbodiesel because only there... between 5500 and 6000 RPM... you actually have superior horsepower. To add insult to injury, high RPMs are very bad for fuel economy. If you give the gasoline engine the RPM it needs to beat the turbodiesel, it will guzzle 40 - 60 % more fuel. Did I mention that diesel fuel is cheaper per litre than gasoline here? But this is a topic for another day. For now, I hope you found this interesting. Congrats on making it this far. Constructive feedback appreciated.
