flat4 - driving performances

 
Driving performance calculations at the example of the 1.2 liter engine 
 
  1. Technical data
  2. Engine power and torque diagrams
  3. Diagramm of tractive forces
  4. Acceleration diagramm
Technical data VW 1200 25kW (34bhp) 
 These are the input values of the calculation program:
 
curb weight 835 kg
cd drag coeff. 0,485
front area 1,85 m²
rolling resistance coeff. 0,0124
tyre rolling radius 30,9 cm
const. drivetrain loss 15 Nm
drivetrain efficiency 0,855
gear ratio 1st gear 3,78
gear ratio 2nd gear 2,06
gear ratio 3rd gear 1,26
gear ratio 4th gear 0,93
final ratio 4,375
Max. engine power 25 kW (34bhp)
Max. engine torque 76 Nm
Max. engine revs 4100 1/min
 
With these input values the 1200cc bug achieves a top speed of 115 km/h and an acceleration from 0 to 100 km/h of 32 sec.

Engine power and torque diagrams 
 

engine power/torque curves
Power and torque curves of the 1.2 liter 25 kW (34 bhp) and  1.6 liter  37 kW (50 bhp) engines. Triebwerks eingezeichnet. The maximum revs of the 1.6 liter engine increased from 4100 to 4600 1/min (+12%), torque increased about 33% due to the greater displacement. Because of the dual pipe intake manifold power increased about 47%. Further calculations are made on the basis of the 1.2 liter flat4 engine.
 
Tractive force diagram (VW 1200 25kW/34bhp) 
This diagram shows the tractive wheel force versus vehicle speed depending on the actusl gear and the uphill gradient. If the gradient is zero the green line shows the rolling tyre force (constant value) and the blue line shows the drag force which is increasing with speed. At the intersection of the blue line and the red line of the 4th gear the 1200cc beetle reaches its top speed of 115 km/h. At this point the full engine power is needed to keep the top speed - no further acceleration is possible.

In the third gear it is possible to climb a 10% slope between 30 and 60 km/h. The offered tractive force is higher than the demand (10% line). Below 30 km/h the 2nd gear has to be taken which enables the driver to climb an uphill gradient up to 20%.

All speeds from stillstand to the beginning of the red curves must be driven with the help of the slipping clutch.
 
tractive force diagram
 
Acceleration diagram (VW 1200 25kW/34bhp) 
The vehicle acceleration versus vehicle speed gives a comparison value between different vehicle concepts with different curb weights, engine torques and gear ratios. What I mean is that the values of power and torque, taken from brochures, don't give sufficient informations about the real driving (and acceleration-) performances..
The VW 1200 acheives in the first gear at 15 km/h a maximum acceleration of 3 m/s² (about 30% of the earth gravity). In the 4th gear at 115 km/h (top speed) there is no further acceleration possible.
If the driver wants to accelerate from 45 km/h with max. acceleration, e.g. when leaving a city speed limit, he can do that in the 2nd, 3rd or 4th gear. In the 3rd gear a 50 higher acceleration is possible than in the 4th. In the second gear even twice of the acceleration of the 4th gear is possible. However the driver has to shift in the 3rd at 50 km/h and the acceleration will soon decrease
.
accelaration diagram

In real life changes in engine power, drag ceofficient and rolling tyre resistance must be taken into consideration. All the calculated forces and accelerations might change when the curb weight increases due to heavy options.  


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