1. Object

(a) To determine the power needed to give the chosen vehicle the required performance; to determine the bottom and intermediate gear ratios.

(b) To arrive at a suitable engine specification for the vehicle in order to give the required performance.

2. Data

(a) Use the data for the vehicle chosen from the following table:

                                               Mass     C_DA     r_tyre      V_max

                                               ___        ____    ____   _____

                                                Kg         m²        m       km/h

1.   Sports car                        1150       0.65      0.28      234

2.   Large saloon                    1600       0.69     0.33      216

3.   Small hatch                        950       0.61     0.27      144

4.   Panel van                         3500       1.50     0.36      126

The mass values given above for an unladen vehicle, expect for the panel van (for which the gross mass is given). An appropriate payload allowance must therefore be made.

(b) For all the vehicles, the following rolling resistance coefficient values may be used, though alternative values may be substituted if though appropriate.

(c) Transmission efficiency: make the appropriate assumptions. One method of estimating transmission efficiency is to assume 97% per meshing pair.

For all the vehicles assume that the gearbox has 5 forward speeds; that the 4th gear is "correctly" matched so that the maximum power speed of the engine corresponds to the maximum possible vehicle speed, and that 5th gear is 25% "overdrive".

For all the vehicles there is a requirement to be able to climb a 36% gradient at the maximum torque speed in the lowest gear.

3. Method

Note that the transmission matching required in this assignment may be determined by plotting either the tractive power or road load power against vehicle speed, or plotting tractive force and road load against vehicle speed.

(a) Select one of the four vehicles given above. The same calculations are required for all the vehicles: only the values of the parameters differ.

(b) Plot the level load curve and determine the engine power required to give the stated vehicle maximum speed.

(c) Choose appropriate values for the following parameters:

• engine type: spark ignition or Diesel;
• a suitable type of combustion chamber and appropriate mean effective pressure at torque peak for the chosen engine type;
• suitable maximum port velocity based in inner valve seat diameter (d);
• suitable valve sizes and hence determine the mean piston speed; check that this is appropriate for the engine duty;
• a suitable value of torque "back-up" in order to determine the mean effective pressure at the power peak.

(d) For the required maximum power determine the required total piston area using:

Power = ¼ Pe A Vp

where: Pe = mean effective pressure at maximum engine speed, as chosen above;
Vp = mean piston speed (at maximum engine speed);
A = total piston area;
& note that the ¼ is replaced by ½ for a 2-stroke engine.

For the required duty choose the number of cylinders and hence the bore size. Determine the stroke to give a suitable engine speed, and hence determine the swept volume.
Is the swept volume appropriate for the class of vehicle and engine type?

Note that an interactive process may be needed to obtain the best compromise between the number of cylinders; the stroke/bore ratio; an acceptable maximum engine speed. The bore and stroke values may also be rounded at this stage, or slightly adjusted to bring the swept volume below a target value (e.g. a tax threshold of 2.0 litres).

(e) Choose an appropriate engine speed at which peak torque will be developed; convert the chosen mean effective pressure values at peak power and peak torque into torque values at the flywheel. Use these values to sketch an appropriate torque curve (on graph paper). Read off torque values from the graph for speeds between peak torque and peak power.

(f) Determine the overall gear ratios for 4th and 5th gears (25% difference).

Convert the engine torque curve to a tractive effort curve, or a tractive power curve in 5th gear, plotting this on the road load curve. (Remember transmission efficiency, which may differ in 4th and 5th). Hence determine the maximum speed of the vehicle in 5th gear.

(g) Determine the first gear ratio by plotting either the road load power or torque to climb the required gradient. Determine 2nd and 3rd gear ratios assuming a geometric progression.

References

H. Bauer (ed.): Bosch automotive handbook, WileyBlackwell; 7th edition, 2007.
W-H. Hucho: Aerodinamics of road vehicles, Society of Automotive Engineers (SAE); 4th Revised edition edition, 1998.