Torque question
Discussion
When a car is out of gear and the engine is spinning freely it is my understanding that very little torque is created (as there is very little resistance)
Lets say a car was de-catted, there is now even less resistance in the exhaust. Would the car stall easier when pulling away (without any throttle)?
I understand there are most likely many variables, but as a genral rule would this understanding be correct, or a load of tosh?
Lets say a car was de-catted, there is now even less resistance in the exhaust. Would the car stall easier when pulling away (without any throttle)?
I understand there are most likely many variables, but as a genral rule would this understanding be correct, or a load of tosh?
At low loads (such as idle) there is practically no exhaust back pressure, as the exhaust mass flow is tiny (compared to fully power). Hence your "freeflowing cat" is doing nothing!
The ability to resist stalling is primarily determined by two main factors:
1) The mechanical inertia of the engine/flywheel. The more mass (or technically the higher the moment of inertia, or "mass that is far away from the centre of rotation") in the system, the slow it will deccelerate for any given force
2) How much "torque reserve" the engine calibration has at idle. Modern engine control systems deliberately run with a retarded ignition angle at idle. This results in less torque, which means less idle speed (because engine friction increases with speed). So, to restore a correct idle speed, the system will open the throttle more. This effectively means the "airside" control is allowing in more air (and hence fuel) than would be strictly necessary to idle the engine if the ignition timing were optimum. But, because ignition angle intervention is "cycle specific" the EMS can easily apply more torque instanly without having to wait for the airside to catch up. Effectively the ignition angle is used as the "fast path" control, and the throttle (airside) as the "slow path" control.
This results in a stable idling system that is able to reject large sudden torque demands without excess engine speed fluctuations. Things like A/C clutch engagement or PAS loads are now rejected virtually seamlessly in terms of engine speed perturbations.
The downside? A slight penalty in fuel economy (for gasoline engines, diesels run excess air so the fuel path is the fast path) at idle / low speed. (The retarded ignition results in heat energy being lost out of the exhaust and less going into the piston as useful work)
Typical ignition angles for a warm engine are approx 15 to 20deg BTDC for MBT (optimum) but a torque reserve ign retard of 10-15deg means the engine will actually run at approx 5deg BTDC when idling.
The ability to resist stalling is primarily determined by two main factors:
1) The mechanical inertia of the engine/flywheel. The more mass (or technically the higher the moment of inertia, or "mass that is far away from the centre of rotation") in the system, the slow it will deccelerate for any given force
2) How much "torque reserve" the engine calibration has at idle. Modern engine control systems deliberately run with a retarded ignition angle at idle. This results in less torque, which means less idle speed (because engine friction increases with speed). So, to restore a correct idle speed, the system will open the throttle more. This effectively means the "airside" control is allowing in more air (and hence fuel) than would be strictly necessary to idle the engine if the ignition timing were optimum. But, because ignition angle intervention is "cycle specific" the EMS can easily apply more torque instanly without having to wait for the airside to catch up. Effectively the ignition angle is used as the "fast path" control, and the throttle (airside) as the "slow path" control.
This results in a stable idling system that is able to reject large sudden torque demands without excess engine speed fluctuations. Things like A/C clutch engagement or PAS loads are now rejected virtually seamlessly in terms of engine speed perturbations.
The downside? A slight penalty in fuel economy (for gasoline engines, diesels run excess air so the fuel path is the fast path) at idle / low speed. (The retarded ignition results in heat energy being lost out of the exhaust and less going into the piston as useful work)
Typical ignition angles for a warm engine are approx 15 to 20deg BTDC for MBT (optimum) but a torque reserve ign retard of 10-15deg means the engine will actually run at approx 5deg BTDC when idling.
I always setup TB'd engine using maximum air/minimum timing and use idle stabilisation by advance for the very reasons stated above. A by product is that is is possible to pull away from a standstill with no throttle or stalling if the clutch is let in slowly enough, this is possible even with a lightweight flyhweel (low rotating moment), generally idel is set using around 2 deg BTDC.
Dave
Dave
DVandrews said:
I always setup TB'd engine using maximum air/minimum timing and use idle stabilisation by advance for the very reasons stated above. A by product is that is is possible to pull away from a standstill with no throttle or stalling if the clutch is let in slowly enough, this is possible even with a lightweight flyhweel (low rotating moment), generally idel is set using around 2 deg BTDC.
Dave
Typically, due to the high leak rate of individual throttles (higer ratio of throttle plate circuference (and hence leaks) to plate area when compared to a single throttle) you pretty much have to do it this way. On really nicely made ITB's it is possible to get them to seal competely when shut, but i find this changes significantly with engine temperature. Also, you have a massive "gain" in the opening area vs plate angle. On my ITB system, 0.1 deg makes approx 500rpm in idle speed when warm. (luckily my DBW system does closed loop idle control ;-)Dave
Depending on shut angle and the gearing of the linkage, the initial open can be softened a little , some TBs even have partial hemispheres (wedges) attached to the throttleplate to dampen the throttle response. The grouping of the load sites can be closed up around this point to facilitate clean mapping of this part of the throttle opening.
Dave
Dave
DVandrews said:
and the gearing of the linkage
Dave
My ITB's are Fly-By-Wire, so i can have any "gearing" i like at any point in the range............Dave
(you can make your car catastrophically difficult to drive if you have a throttle plate that initally opens, then closes, then opens again with pedal position :-)
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff