For example, a regular grocery-getter with enough torque just to get around simply rolls off when in D and at minimum revs.
Why do super or hypercars not just take off when the same is done? I would assume that the amount of torque at the same rev range would be significantly higher for example in an Agera versus an Accord.
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First, idling an engine is not producing *that* much power – just enough for keeping the air conditioner going and electrical systems powered. Second, the transmission, be it a torque converter or dual clutch, modulates the amount of torque transmitted from the engine to the drivetrain to allow the car to creep at a normal speed.
The force transmitted by a torque converter in automatic cars depends on the slip. This is the difference between RPM and (geared) wheel speed. A stationary car has a wheel speed of 0, and a fairly low slip given by the engine speed.
At idle, the force is low. Increasing the RPM increases the slip and thus force. A stronger engine can achieve a higher slip when you floor it, and so higher acceleration, without any more force during idle.
Engines at idle RPMs do not make that much torque, regardless of whether they are a ‘supercar’.
When you read torque values on spec sheets, usually the manufacturer is picking the highest value across the whole RPM range. Usually the torque value they select is going to be between 2000 to 7000 RPM. Below that range would read much lower torque.
Gasoline engines are very poor at making power at low RPMs. If you got the actual torque value from a super car with an engine at idle speed, it would still be on the order of a big riding mower.
Additionally, modern engine control systems estimate and then control the torque produced by the engine in real time. So even if an engine could produce a high torque at idle, the control system could easily modulate its properties for drive-ability.
Interestingly, electric cars must deal with this problem because they CAN produce full torque at near zero RPM. If the torque were not controlled, it would be super easy to spin the wheels at every traffic light. Fortunately, this is easy to modulate with current limiting.
In some really fast cars (expensive or not), not really, the torque converter that sits between the engine and transmission will engage at a higher RPM than moms grocery getter, say at 2500 RPM instead of 1100 RPM, so if the car idles at 1500 you have to hit the gas until the engine hits 2500 RPM to start moving. It might creep slightly but it’s weird when you let off the brake and it doesn’t go anywhere.
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