Torque Curve : The Engine’s Heartbeat
At the center of engine power, there is a force called torque. This is a rotational force that we can plot on a graph that usually presents itself as a curvy line. This is called the torque curve.
The torque curve is the line on a torque graph that shows the rise and fall of the engine’s rotational force due to the design and limits of its physical parts.
Two major units of measurement are used to measure torque. Pound Foot (lb-ft) and Newton-metre (Nm). A pound-foot is the expression of a pound of force acting on a foot of leverage. Newton-meter is one Newton applied at a meter of leverage. Both mean the same thing in terms of using distance and force to influence a center point. The further you are from the center point the more leverage or opportunity for more force now exists. This article helps you understand what a torque curve is and why it presents as a curve when plotted on a graph.
What you will learn:
Torque Curve and Engine Performance
Since torque is a rotational force we can see anywhere on a car that turns as a result of the engine having torque. Torque is developed in the engine and then transferred out to different parts of the car through gears and connections. All these different parts moving/rotating as a result of the engine is what is called mechanical power. Mechanical power starts with how the engine operates. Most automotive engines are IC (internal combustion) engines that operate on a four-stroke combustion cycle. In an engine, pistons are moving up and down in a chamber/cylinder and each movement up or down is one stroke. The pistons are connected to a crankshaft which moves in a circular motion. With each rotation, it helps the piston move up and down. Out of the four-stroke cycle, the first stroke that starts the cycle is the one going down. This first down creates a vacuum, sucking air in a mixture of fuel. The second stroke upward compresses the mixture and is ignited. The third stroke is the mini-explosion pushing the piston down. The third stroke is called the power stroke. This stroke creates enough force to turn/torque the crankshaft. The fourth stroke pushes out the exhaust gases from that combustion.
However, the amount of air that gets sucked in on the first stroke determines how much force will be produced on ignition. When you’re using your acceleration pedal you are creating an opening for more airflow. The more air gets sucked in the more fuel is added for a good mixture and the force you have on each ignition, then the faster you go. When pressing the acceleration pedal you create a max opening for airflow on a part called the throttle body. This part is very responsive so it happens in an instant. The engine will start to rev higher because it now has more air to work with. Revving higher means the four-stroke cycle is happening faster and faster. There is a point where the engine is revving so fast it is maximizing the amount of air that is getting sucked in and is only restricted by how much air that can squeeze through the throttle body.
Torque Curve: Understanding its Importance
A torque curve visually represents your engine’s capability and limits. When plotted on a graph, as your engine revs it starts to move faster and faster revving higher and higher. The revs (revolution) refer to the rotation of the crankshaft. Usually, if you look at your gauge cluster, your rev counter has numbers from 1-6/7 maybe higher with a redline at the end. These numbers are times a thousand so when your needle indicates 1 is 1,000 2 is 2000 or 3 is 3000 RPMs (revolutions per minute). When your engine is accelerating it’s gradual because it takes time to create power, not much but it will be a gradual rise instead of an instant. This explains the rise of the torque curve line in the beginning.
At a certain point, the line reaches its peak. This is the most amount of air your engine can take to create power. Following the torque curve this is where the line gradually starts to descend. This doesn’t mean your engine is losing power. The descent is more so showing the limits of certain engine parts. Engine parts like the throttle body and intake valves for example. The size/diameter of both parts will limit the amount of air flowing in. As your engine revs higher it is sucking in more air, but if the amount is limited the piston is essentially choking for more. This means there is a limit to your engine’s torque which is dependent on your engine’s design and how it is calibrated.
Torque Transfer to Wheels
Your engine converts torque from the crankshaft to the flywheel. The flywheel is the connection between the crankshaft and the transmission. Your transmission is how you can utilize different levels of torque coming from your engine whether it’s an automatic or manual transmission. Your transmission also is the reason you can stop your vehicle with the engine still running. This is where the clutch comes in, for a manual vehicle. An automatic transmission vehicle uses a torque converter. These are units that are a part of your transmission system that allow the engine to be disengaged from the transmission.
This is what allows your wheels to stop while the engine is still spinning. Your transmission uses different sizes of gears like a bicycle to manipulate the rotational forces coming from the engine. These different sizes are what we call the gear ratio. Bigger gears yield more torque but low speeds whereas smaller gears yield high speeds but low torque. Connected to the transmission is your driveshaft which is ultimately driving your wheels. One thing to note is that through all these connections from your engines, there are parasitic losses. This is a loss of power due to friction or heat. Gears working together have friction and heat which will result in parasitic losses along the way to your wheels.
Applications of High Torque Engines
Remember that the more distance you have when trying to turn a wrench the more power you can apply to whatever bolt or screw you are trying to get out. So engines calibrated for high torque are mostly used in towing, hauling off-road, and truck use applications. These types of applications will benefit more from high torque making the job a lot easier. When towing, hauling, off-road, or truck driving the wheels on the ground are carrying a lot of weight or could be stuck or even both. The wheel being hard to move is no different than that bolt or screw you are trying to get out, especially if the bolt/screw is stuck. The more pound-foot of torque you have the easier it will be to get that wheel/bolt/screw moving.
Conclusion
When buying a vehicle it is important to know what you’re aiming for, and what type of use you’re looking to get out of this vehicle. This is why it’s important to understand what torque is and how to understand torque curves. Torque curves help us to understand how, where, and when an engine performs. You see where the low-end torque is, and how that is an advantage to you. All of this informs you about your buying decision whether it is the vehicle or upgrades.
FAQs:
What is a Torque Curve?
A torque curve visually represents your engine capabilities at different RPMs. It is a graph that is key to understanding how your engine produces power.
Why do engine torque curves vary?
Engine torque curves vary because not all engines are built the same. A torque curve is a direct reflection of an engine’s design and its intended use. One thing to note is that an engine with a high torque output can’t rev as fast as engine engine with a high horsepower output. The engine parts are designed and calibrated to produce either more rotational force (torque) or be as lightweight and nimble as possible to rotate faster. Therefore when looking at torque curves for different engines they will vary because torque is utilised differently.
What does a flat torque curve mean?
When looking at engine torque curves the peak of the curves represents the engine’s peak torque at a certain rpm. If that peak is extended and looks more flat it means the engine is producing maximum torque for a longer rpm range.
Does higher torque mean faster acceleration?
Higher torque is one aspect of quicker acceleration. High torque helps the start of acceleration. For quicker acceleration to continue that is high horsepower. Horsepower is work an engine can do over time so if the engine does more work in a shorter time, that’s more horsepower. Torque is the rotational force that gets you moving, especially from a standstill.
