The rotational masses influence the dynamic behavior of the vehicle significantly. Please see detailed information in the attached publication. In case of the mentioned example, the engine & flywheel may represent main rotating components, but the tires should also be considered because of their masses. Please consider, that the inertia of engine and flywheel have to be transformed to the wheel-axis under consideration of the transmission ratios. 

If you have the vehicle coast down data, you need information about two cases:

1.) vehicle with open clutch of flywheel and engine, so that these components are not in operation

2.) vehicle with closed clutch of flywheel and engine, so that the drag resistance of these components is considered. In this case, the engine braking behavior is considered with all engine related factors, e.g. engine friction and engine braking behavior.

The difference between 1.) and 2.) gives an estimation about the influence of the rotational components on vehicle dynamics.

Kind regards,

Mario

Conference Paper Basics of longitudinal vehicle dynamics

Hi Mario, Thanks for the document.

Please the observations:

1. The subject vehicle with me is having an empty weight of 12500 kg and is a city bus & this gets me a very high value of mass due to rotational components and is around 18000kg for 1st gear with ratio 6.658 & decreases to below 1000kg for 6th gear with ratio 0.77.... Is this Ok for large vehicles

2.I applied these value to vehicle logitudinal equations but yields bad results for deaccelerations. 

3. Is there any other method to cross verify the inertial mass of rotational components. Like if I use the total resultant force on the vehicle to be used for getting a Torque value using wheel radius and then use longitudinal acceleration for calculating angular accleration and then divide Torque with Angular accelration to get total rotational inertia. ??

4. During negetive acceleration...what is the behaviour for inertial mass of rotating components?? what is the longitudinal vehicle equation to be used for coasting and negetive acceleration without braking??

Regards,

DEEPAK

ad 1.): Make sure, that you have added the tranlational masses in a right way, so that all moved components of the vehicl are considered. To compute the influence of rotating masses, try to simplyfy the problem by fokussing on the main masses (e.g. flywheel and wheels only in a first step). I cannot verify your values, because of missing information about flywheel, engine and transmission.

ad 2.): If bad results means, that the acceleration is too high: Consider reasonable driving situations, e.g. the proper gear for a certain speed.

ad 3.): Basically, it is rather simple to estimate the inertia of rotational components. You can get information out of CAD-models for example. I recomment to simplyfy the problym by reducing the number of components.

ad 4.): The inertial mass of rotating components remains the same during braking, but the transmisstion ratio might change, so that the effect "at wheel" might change.

The longitudinal vehicle equation for coasting at horizontal road (without braking) consists of the air drag plus the rolling resistances equal the deceleration multiplicated with the total vehicle mass. The total vehicle mass includes the translational masses plus the computated effect of the rotationalmasses at the wheel axis.

Kind regards,

Mario

Hi Mario..

I am trying to estimate the mass of a vehicle with the use of simple longitudinal equations for acceleration:

(M+Mr)*a= Ftractive-Faero-Froll-Fgrade

M- Actual Mass in kg

Mr-Mass of rotational components

a- Acceleration(longitudinal) calculated using Velocity from a front axle wheel speed sensor

Ftractive- Total tractive force on wheels using Actual engine % torque minus the frictional torque in CAN message

Faero- Aero drag resistance

Fgrade- Test conducted on no grade and logs are being used to validate the equation above in a spreadsheet from CAN logs

Froll- Rolling resistance

Now-

Q1. I use Torque values minus nominal friction -Is this okay when am using the rotational mass separately in the equation??

Q2.The actual mass is known to me as 16200kg loaded bus for test on no grade flat track. Rolling resistance verified from COast down test and aero drag also tested..

But when  calculate the rotational mass from known values of Moment of Inertia of engine(0.53 kg sqm),Flywheel(0.45),Clutch(0.05*2),Gearbox(0.4),Tire and axle drve(including prop shaft- 101)...I get values of Rotational mass for 1st gear as 18000kg, 2nd gear 10000 kg and so on...

These values when compared with collected log data and calculated using Vehicle dynamics equations vary by max +-2000kg which is Ok i guess

BUT when using the equations for negative acceleration values, the Mass calculated is much less than actual vehicle mass of 16200kg proving the inertia calculations wrong..

How does inertia forces behave in negative acceleration.

Please help correct the equation above if I am missing any component...

regards,

DEEPAK

ad Q1.: Its OK to use the Mr seperately. Important is, that MR is calculated right, by considering the Moments of Inertia and the transmission ratios.

ad Q2.: I did not calculate the values for a bus with a flywheel yet, but in case of a standard car, the value of Mr lies in the area of about 5% in the highest gear and 60% in the lowest gear in relation to M. In your example, the maximum value of Mr (18.000 kg) is about 110% of M, which might be reasonable because of the lower gear ratios in busses, the heavier drivetrain and the flywheel. 

In case of negative acceleration, you have to consider, that the effect of some driving resistances is negative, that means that the ineria of translational and rotational components drive (!) the vehicle. You have to switch the sign of these terms, so that they correspond with the physical behavior.

At deceleration of your model (horizontal road, switched of engine power), the driving equation includes the following components:

air drag, rolling resistance ... evoke forces against the driving speed direction

forces and moments of inertia ... evoke forces in driving speed direction

if applied: braking forces ... evoke forces against the driving speed direction

Kind regards,

Mario

Thanks for the Information.

This means during coasting, the inertia of a vehicle can be used for regeneration in a hybrid vehicle?

Regards,

DEEPAK

Yes.

Kind regards,

Mario

Hi Mario,

While using the Torque value available on CAN for measurement of tractive force, I have been using the Actual Engine Percent Torque value minus the Nominal friction value for getting the effective torque. From the final torque value after this I subtract or add motor torque based on regeneration or assist mode of  motor. 

Since the Frictional torque required to keep the engine alive is being subtracted from the Engine Torque, does this also account for torque or rather power loss due to inertia also??  Since I am using Inertial mass for rotational components separately also...do I need to stop subtracting the Nominal friction values from AEPT value??

Again after correcting the longitudinal equation, I intend to use it as a state space model for applying Extended KaLman filter for estimating grade and mass...This is the reason I want to correct the equation as far as possible to get a good value for initialization for error co-variance matrices for the filter...I am conducting few more coast down tests also...

The current difficulty I face is the unavailability of any equipment to Measure the required moment of inertia. So i rely heavily on the coast down test results.

Warm regards,

DEEPAK

Dear DEEPAK,

this procedure sounds good. Here are my remarks:

- Make sure, if the torque value available on CAN consideres the engine friction influence or not.

- In case of vehicle coasting, the engine resistance behavior which has to be considered is composed of the mechanical friction plus the losses of driven 4 stroke cycle.

- The friction always reduces available energy - during driving as well as during coasting the vehicle. In this way, you have to apply  the friction related term as a substractive part in the energy balance.

- You can estimate the moment of inertia by simplification of the main components, e.g. engine, transmission, flywheel, wheels. Flywheel should be easy because of the rather simple geometry. Engine and transmission are more complex: Perhaps you will find some estimations in literature. If not, you could build up simplified geometry models of the main parts in CAD (e.g. crank shaft engine flywheel, reduced masses of conrod and piston, camshaft (consider the gear ratio of camshaft), main shafts of the transmission). In the CAD model, you can compute the moments of inertia.

Kind regards.

Mario