Non-Clutch Vehicle Drivetrain - How Does it Work?

I am making a vehicle simulation, but just noticed a big issue - there are a lot of cars with no clutch in them, but I haven't driven a car with automatic/clutchless-manual gearbox, nor been even a passenger in such a car. Consequently, I have no idea how such vehicle drive or operate.

For a vehicle with a clutch, I know how it drives and I also know the physics behind it operation - torque from engine gets transmitted through the clutch to the transmission, then to differential and then it reaches wheels, the wheels generate torque from ground friction which goes the other way around and slows down the engine. If the clutch is disengaged, then the engine won't transfer torque. If the clutch is partially engaged, then some of the engine torque will be transmitted and the rest of it will be lost in some form of heat during clutch slippage.

However, I have no idea how vehicles with an automatic gearbox or a semi-automatic gearbox (e.g. sequential) work. As far as I know, they don't have a clutch. However, I've seen that such vehicles can have their engine revving while not transmitting any torque to the differential.

Is it that they're directly connected to the transmission or is there something else there? What I am asking for is some sort of guidelines for me to figure out how to calculate their maths (plus, if possible, some equations would help).

• Do you mean no clutch pedal or no clutch in the drive train? Commented Mar 16, 2017 at 17:48
• I mean no clutch in the drive train. Commented Mar 16, 2017 at 18:20

Conventional automatic transmissions have a torque converter which uses a viscous rather than a rigid coupling to connect input and output. This performs a similar function to a friction clutch but doesn't require manual engagement. Also automatic gearboxes are based on epicyclic gear trains so all the gears are permanently meshed with each other and ratios are selected by locking different combinations of the sun, planet and annular gears so there is no need to match different shaft speeds to each other.

Sequential gearboxes use a combination of straight cut (as opposed to helical) gears and dog clutches with gears engaging by sliding along the input and/or output shaft so the gears never go fully out of mesh with each other but rather are either locked to or freely spinning on their respective shaft. Again this means that it is not necessary to match input and output speeds, however a clutch is still required to start from a standstill. Motorcycles, for example typically have sequential gearboxes and hand operated clutches.

The more modern type of auto/semi-auto gearbox typically uses twin wet plate clutches to change between pre-selected gears in this case the gear selection and clutch engagement is done by actuators which are controlled by a computer. In many cases they can be operated as fully automatic or semi automatics where the driver shifts up or down by operating a paddle or lever. So while they do have a clutch is it not manually operated to change gear. Although, as in a sequential gearbox, manual clutch operation may still be required to start from a standstill often this will be an additional paddle or paddles. Although many semi-auto transmission will also incorporate an anti-stall system which automatically releases the clutch if the revs drop below a certain level at low speed.

So from the perspective of the driver there are 3 main cases.

• Full auto (either conventional auto or modern semi auto in full auto mode) : press accelerator pedal to go and the brake to stop and the clutch (or equivalent) sorts itself out automatically eg with automatic transmission if you brake to a stop the engine won't stall so no manual clutch operation required.

• Sequential/semi auto : manual clutch operation is required to start/stop but not to change gear. Often this will be a hand clutch.

• Manual : manual (usually foot pedal) clutch required to start and to change gear.

These cover the most common systems, although there are various other transmission types around.

Sequential gearboxes are most common on motorcycles and some classes of competition cars (especially those heavily adapted from road cars (eg rallying).

Most conventional road cars have either conventional automatic or manual transmissions.

Computer controlled semi-auto/auto transmissions are mostly found on high-end sports and racing cars eg F1 and GT formulae.

There are also automatic transmissions which allow for an approximation of manual gear selection eg the Porche Tiptronic system and it is also possible to automate a conventional manual transmission, although this is a lot of complexity for no real gain.

For the sake of completeness there are also centrifugal clutches which only engage above a certain engine rpm but these are only really found on things like chainsaws and self propelled mowers. Also steam piston engines don't require a clutch as they can develop torque from zero rpm.

It is also worth noting that for a driving simulation, unless you have really good haptic feedback in the control hardware it will be very difficult to simulate the feel of a manual clutch.

• So it's like clutchless when shifting, but it acts similar to clutch during startup? Like disengaged before starting to drive and than engages when "triggered" to drive. I guess I could do an approximate simulation using simplified clutch principle then? OK! Oh, and sequential, even though working differently, still can be approximated the same way, right? Commented Mar 16, 2017 at 20:23
• short answer is yes, I've edited my answer to add more detail but in essence pretty much all vehicle transmission have some analogue of a clutch which allows the engine to run while the vehicle is stationary. Commented Mar 17, 2017 at 0:52
• OK! Thanks for the answer. This explains how it works. Now I just have to figure out some maths and try to do more research to simulate it as realistic as possible;) Commented Mar 18, 2017 at 17:52

There are 2 important human functions that an automatic transmission must replace:
1.The driver chooses a gear that's appropriate for the torque and speed requirements of the vehicle. In an automatic transmission, shift schedules are stored as breakpoints for upward and downward shifting, usually based on engine torque and speed.
2. The driver feathers the clutch to shift smoothly between gears. The automatic transmission incorporates a hydraulic torque converter, a fluid coupling between the engine and transmission, which damps any jolts in engine torque or speed. This animation is very useful: