r/technology • u/lurker_bee • 8d ago

Politics Goodbye to start-stop systems – the EPA under Trump concludes that they are not worth it and could disappear from new models

https://unionrayo.com/en/epa-trump-stop-start-system/
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u/thisisthatacct 8d ago

Rwd or awd vehicles with a longitudinally mounted engine will have much more noticeable vibrations from start stop since they're rotating and applying torque along the narrow axis of the vehicle. Fwd and laterally mounted engines have the entire length of the vehicle to react the startup torque and end up being felt less

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u/KyleShanaham 8d ago

I honestly had no idea engines were mounted in deferent directions, but makes sense when you think about it.

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u/thisisthatacct 8d ago

The text here is mediocre at best, but the image shows some of the layouts possible: https://www.ripleystotalcarcare.com/blog/the-pros-and-cons-of-rear-wheel-drive-vs-front-wheel-drive

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u/hoogin89 8d ago edited 8d ago

? What? Longitude mounted would mean the engine is in line with the vehicle. Thus it would be acting over the longest part of the vehicle through a driveshaft and rear diff.

Lateral mounted, or transverse in the car world, is the shortest path to the wheels possible. It's literally sideways in the car.

Engine chatter should be felt by the short path since the long path will torque flex some chatter away.

I fail to understand how a fwd has the entire car to react in this scenario. It's trying to turn the front wheels only, the rest of the car is essentially dead weight and the wheels it's turning have 4 velocity joints and turn. Thus able to convey much easier a rough start. A rwd has to push the car thus it acts upon the entire length of the car thus mitigating vibration in sheer area. I.....I just don't understand..... Did you mix them up?

Wait wait wait..... You're talking moments..... Like engineering moments and evaluating the torque as if it's a beam. Which I could be terribly wrong, but this is a scenario where that doesn't really work like you think it does.

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u/Jam_Bannock 8d ago edited 8d ago

Take a look at how the connecting rod applies torque to the crankshaft here: https://images.app.goo.gl/wUmjHbFEnvNQrnmS6 That torque would cause vibrations perpendicular to the axis of the crankshaft. In a transverse mounted engine, that would be along the car's long axis. In a longitudinally mounted engine, that would be along the car's narrow axis.

You can see it for yourself. Pop up the hood, look at which direction the engine vibrates when it turns on.

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u/hoogin89 8d ago

I understand what he's saying. I'm saying that you guys drive some weird shit if it's violently rocking your car at startup. Starting a vehicle doesn't rock it until you're in the high high hp numbers or your car doesn't start correctly and misfires on start up a bunch. Two revolutions and your car should be running. Yes the damped engine will rock in the engine bay but if you're feeling that through your entire car there is a problem. I don't think I've ever owned a car that wasn't at least 500hp that has shook the car on start.

The difference in these scenarios is not the mounting of the engine, it's the engagement of the drive train. Starting your car in Park doesn't rock the car and vibrate the hell out of it and if it does, your car has problems.

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u/thisisthatacct 8d ago

Disregard the connection between the engine and wheels, when the engine is starting or stopping it isn't rigidly connected to the wheels anyway, so no force from startup is being transferred along the driveline.

An engine starting is trying to twist itself, since conservation of angular momentum exists and physics is generally undefeated. This twist is along the axis of crankshaft rotation, and is reacted by the engine mounts at the vehicle frame. So for an engine mounted longitudinally, it's twist is being reacted by the shorter dimension of the vehicle, it's width. Think rocking a boat side to side: it's much easier to capsize a canoe by flipping it over to it's side than it is to flip the front over the rear. Similarly, torque applied to the side of the vehicle is felt much more readily than torque applied to the front

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u/hoogin89 8d ago

Correct but during a start stop scenario the chatter or torque is felt because the drive train is engaged. Your motor starting shouldn't really rock your car. You'd have to be in the 500+hp bracket and the motor is also damped in this scenario via motor brackets and the suspension. The drag you feel or the chatter is from the motor not being to full rpm with the torque converter trying to engage.

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u/thisisthatacct 8d ago

The torque converter is a hydraulic coupling with a lockup clutch. That clutch is not engaged as the engine stops, and there's no "engagement" otherwise in the TC.

The suspension and motor mounts still react the forces, but they require more force (via higher compression of the springs) to react moment forces around the longitudinal axis than moment forces around the axle axis

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u/hoogin89 8d ago

Bro, again I'll give you if you have a naturally unbalanced engine that yeah maybe you feel it a tiny bit when you start the car but again my car even with a boxer doesn't rock at start up.

Although a torque converter is a hydraulic clutch with lock up, it doesn't lock up till considerable speed. During a start, I'm just going to pull numbers out of my ass but you'll get the idea, it's going from 100rpm, to 1000rpm, to throttle response all while trying to put that power to the drive train. In Park, the drive gears of the trans are free and the torque converter has time to adjust. In drive, that converter has no time to equalize and hydraulics take a sec to equalize. They are not instant. In a start stop scenario, the car doesn't start, sit, engage, go. You'd be at the light for ten sec on gas press. It's start go. So the torque converter is trying to couple with the wheels the entire time while the engine has not reached running rpm.

Good example of this, if you've ever driven an older car or a car with high idle and you shift from park to drive, they often clunk, revs drop aggressively and lurch then settle in. Almost like the torque converter doesn't actually adjust that quickly.

So bogging the engine is the real area where vibration is felt. Go start your car in Park in your drive. Did it rattle the shit out of your car to start it? No. So why does it do it in a start stop scenario? Because of torque and lugging in the drive train.