Hybrid Drive

Research

Reducing Emissions from Motor Vehicles by Using Hybrid Drives By Friedrich Marker, Faramarz Jamaly and Julian Zycinski

has to change until a new balance of torque is achieved. Such operating conditions are described as unsteady.

The following article describes the technical and economic aspects of hybrid vehicles as well as their potentials for future application. It discusses why, under present circumstances, more importance should be placed on the series production of hybrid vehicles.The company Intec Engineering, as a developer of components for internal combustion engines, has set the target of showing ways in which the cars of the future can be modified to become hybrid vehicles. Subsequent series production will then complete this objective target. Moreover, Intec Engineering is working intensively on the subject of thermo-management in hybrid vehicles, in order to further optimise the reduction of exhaust emissions.

The operating conditions under which the engine achieves its best degree of efficiency or its maximum torque may be read off directly from the performance graph. The engine output Pe is not displayed directly in the performance graph but may be calculated with the aid of the equation Pe = M · ϖ and ϖ = 2 · π · n. The maximum output is achieved by the engine at the operating point at which the product of the torque M and the speed n reaches its highest value, Figure 1.

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Introduction

When favourable characteristics of different systems are combined, the result is known as a hybrid. In the context dealt with here, the combination of an internal combustion engine and an electric motor to power a motor vehicle is called a hybrid drive system. Using a sensible combination of these two types of drive, as well as intelligent engine management, the advantages of both systems may be utilised. This will lead to a considerable saving in fuel and thus to a corresponding reduction in the emissions of exhaust gases, especially in the areas of traffic congestion and high population density which are nowadays highly polluted. Vehicles in the small and mid-size ranges in particular are suitable for this type of drive, these being the ones that are mainly operated in urban traffic under conditions in which internal combustion engines achieve an unreasonably low degree of efficiency. In the following, this subject will be explained and the reasons for it given.

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Effi ciency

Modern internal combustion engines of the size considered here achieve effective degrees of efficiency of ηe ≈ 35%. This maximum value is only achieved at one single

ATZ Automobiltechnische Zeitschrift 103 (2001) 1

operating point, at which the engine is running under a high load at a speed within the maximum torque range. In the higher speed range and with a lower load, the efficiency drops sharply. When idling, an internal combustion engine consumes about 1 litre of fuel per hour just to keep itself running and without any output to the outside. At this operating point, engine efficiency ηe = 0. The relationship between engine load, engine speed and the degree of efficiency achieved is determined on engine test beds and clearly shown in the operating performance graph.

If an internal combustion engine is operated in the usual way in combination with a manual or automatic transmission as the power source of a motor vehicle, the velocity of the vehicle v is proportional to the engine speed n. This proportionality factor changes with the gear selected. The way in which the engine and vehicle interact may be seen when we plot the resistance curve of the vehicle and enter it into the engine performance graph. In order to determine the resistance curve, the driving resistance Fw of the vehicle as a function of the vehicle velocity v must first be calculated. This force is the sum of the rolling resistance FR and the air resistance FL. On inclines, the downward force due to the slope FH is added.

To understand this better, in the above operating performance graph, the engine torque M over the engine speed n with an effective efficiency of ηe is shown as a parameter instead of the generally-used effective pressure pe. Instead of the efficiency, the specific fuel consumption be with the unit kg/kWh is often employed as a parameter; its reciprocal corresponds to the effective degree of efficiency.

While the rolling resistance is only dependent on the mass of the vehicle m and the value of rolling friction μR, the air resistance increases with the frontal area A, the coefficient of drag cd, the air density ρ and the square of the vehicle's velocity v2. The downward force due to the slope results from the mass of the vehicle and the angle of incline α (the angle between the road surface and the horizontal).

By changing the accelerator position, the engine may be operated at any desired point of its operating performance graph at a constant speed if the torque produced at the time is taken from it by a connected drive. In such a case, the operation is described as steady-state. If the torque taken from it does not correspond to the torque produced by the engine, the engine speed

You will nd the gures mentioned in this article in the German issue of ATZ 1/2001 beginning on page 58.

Emissionsreduzierung von Kraftfahrzeugen durch den Einsatz von Hybridantrieben

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Hybrid Drive

The following relationship exists: FW = FR + FL + FH = m ⋅ g ⋅ μ R ρ + A ⋅ c d ⋅ ⋅ v 2 + m ⋅ g ⋅ sin α 2 Eq. (1) FW = m ⋅ g ⋅ ( μ R + sin α ) ρ + A ⋅ cd ⋅ ⋅ v 2 2 Even at low angles, the downward force is considerably greater than the rolling friction of the vehicle. On downward slopes, it becomes negative, i.e., it works in the opposite direction to the other types of resistance. If a vehicle is to move at a constant velocity, the force of resistance Fw must be compensated for by a driving force of the same magnitude FA = - Fw, so that the condition of equilibrium is fulfilled. This force FA results from the torque of the driving axle MA. At the drive wheels with the wheel diameter dR, this torque is transformed into a pair of forces, Figure 2. FA =

2 ⋅MA dR

Eq. (2)

The force Fa operates as the propulsive force on the axle, while the other components are absorbed by the force of static friction between the tyres and the road surface. If the condition FA = - Fw is not fulfilled, the velocity of the vehicle will change. Since the travelling velocity is connected to the engine speed, the operation of the engine will become unsteady, until the above condition is fulfilled again. In order to produce the resistance curve, in addition to the force of resistance, the wheel diameter dR and the overall gear ratio iges between the engine clutch and the drive axle must be known, i ges =

n Motor = i Getriebe ⋅ i Achse n Achse

Eq. (3)

so that the engine speed and driving velocity may be put into context. The ratio is different for each gear selected, so that different resistance curves are produced corresponding to the different gear ratios. If a vehicle is to achieve as high a maximum velocity vmax as possible, then the total gear ratio iges must be selected in such a manner that, at this velocity, the engine reaches the speed nPemax at which it produces its maximum output Pemax. There is a connection between the force of resistance Fw, the velocity v and the output P required to overcome the resistance, and this is described by the equation P = Fw · v.

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Starting from the maximum engine output Pemax and the data of the vehicle already known (mass m, rolling friction μR, frontal area A and the coefficient of drag cd), the top speed achievable on a level road vmax may be calculated. Taking into account the degrees of efficiency of the gearbox and the differential ηS and ηD respectively, the equation Pemax ⋅ η S ⋅ η D = FW ⋅ v max = μ R ⋅ m ⋅ g ⋅ v max ρ + A ⋅ c d ⋅ ⋅ v max3 Eq. (4) 2 applies, from which the unknown quantity vmax may be iteratively calculated. From the final velocity vmax, which is now known, and the wheel diameter dR, the required speed of the drive axle nAmax may be calculated. Since the circumferential velocity of the drive wheels corresponds to the velocity of travel, the following relationship applies: n Amax =

v max π ⋅ dR

Eq. (5)

With the known engine speed nPemax at which the maximum power is achieved, the overall gear ratio iges is thus fixed. i ges =

n Pemax n Amax

Eq. (6)

For the gear in which the final velocity is achieved – in most cases 5th gear – every engine speed n may now be allocated to a driving velocity v. v=

n ⋅ π ⋅ dR i ges

Eq. (7)

The resistance curve may now simply be calculated point for point and transferred to the performance graph. First, the driving velocity v is determined (Eq. (7)) for an assumed engine speed and then the force of resistance Fw to driving (Eq. (2)). The required engine torque M, when taking into account the degree of efficiency of the gears ηS and ηD respectively, finally results from the following equation: M=

FW ⋅ d R 2 ⋅ i ges ⋅ η S ⋅ η D

comes greater owing to the downward force due to the slope. Very often, the ratios are made to be somewhat higher. This means that the vehicle reaches the final velocity in a shorter time, and reacts to headwind components and smaller slopes with low losses in speed. The highest velocity possible is no longer achieved when this type of gearing is employed. As the operating performance graph shows, the torque required at the speed producing the optimum degree of efficiency is still very low, and the operating point is far removed from the optimum degree of efficiency. It is not sensible to alter the whole gear ratio in order to raise the resistance curve, since then the engine, when operating, would only be able to use its full power in 3rd or 4th gear, depending on the gearbox design. This would result in the design of a gearbox with very wide ratios and one or two gears that could only rarely be used. The 4+E gearbox offered by VW some years ago was a step in that direction. Vehicles fitted with this gearbox achieved their top speed in 4th gear, while the E-gear achieved lower fuel consumption but was unable to achieve any acceleration worth mentioning even on a flat and level track. This gearbox was not accepted by the customers due to its unelastic behaviour. The above considerations show that the best degree of efficiency of the engine cannot be used in a steady state. The engine can only be operated in an unsteady condition in this point during acceleration. Due to the surplus torque, the vehicle velocity and engine speed increase. This property is shown in the performance graph by the dotted line (Figure 1, dotted lines). The resistance curves for the other gears with the selected gear ratios may be determined analogously. Since the total ratios become higher and the attendant speeds

Eq. (8)

The resistance curve produced in this way applies for 5th gear, in which the maximum speed is achieved. It indicates the engine torque M necessary for steady-state operation for the various speeds in this gear. Its end must run through the point in the performance graph at which the engine achieves its maximum output. In uphill operation, the resistance curve shifts to higher values, since the force of resistance Fw be-

Forces to the wheel

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Hybrid Drive

drop, these curves are at an even lower engine torque and thus even further from the operating point with the best degree of efficiency. As a comparison, the resistance curve for 1st gear was also entered on the operating performance graph. In the calculation, the gear ratio of 1st gear was taken as being i1stG = 4 and of 5th gear as i5th G = 1. At the same engine speed, the vehicle speed is thus reduced to one quarter.

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Urban Traffic

Engines in vehicles that are used in urban traffic or are stopped in a traffic jam have degrees of efficiency that are markedly below 10%. In this unfavourable "stop-andgo" operation, the driving power is required for overcoming the resistances to driving and additionally for accelerating the vehicle. This unsteady operation requires a higher engine load, which, due to the low gears used, only improves the degree of efficiency to an insignificant extent and temporarily. Part of the kinetic energy may, with the appropriate driving manner, be used to overcome the driving resistances in overrun operation, but the greater part is transformed into heat by the braking process and is thus useless. The correctness of the above considerations may be proven by means of a simple calculation. For this, the efficiency of a vehicle that achieves a consumption of B = 5 dm3/100 km at an average speed in urban traffic of vm = 30 km/h is determined. The following data are taken as a basis for this vehicle: Frontal area A = 1.5 m2, coefficient of drag cw = 0.3, mass m = 800 kg and rolling friction value μR = 0.01. For this vehicle, the calculation (as per Eq. 1) produced a mean driving resistance Fw = 97 N. To overcome the driving resistance over a distance covered of s = 100 km, work to the amount of WNutz = Fw . s = 9,700 kJ is necessary. For this, together with the fuel (Hu = 40,000 kJ/kg, ρ = 0.75 kg/dm3), the heat generated by the fuel Qzu = Hu · B · ρ = 150 MJ is used. This produces a degree of efficiency of ηe = WNutz/Qzu ≈ 6%. The observations carried out above show that all improvements to the degree of efficiency of the engines and of the drag coefficients as well as reduced vehicle masses can only lead to slightly reduced fuel consumption. In the case of conventional vehicle drives, internal combustion engines achieve very poor degrees of efficiency due to the unfavourable operating conditions. Because of their CO2 emissions, they con-

ATZ Automobiltechnische Zeitschrift 103 (2001) 1

tribute considerably to the pollution of the environment, and consume far greater quantities of valuable fuels than ecologically justifiable.

4

Comparison Internal Combustion Engine – Electric Motor

The shortcomings described here are, however, combined with considerable advantages, which have resulted in the fact that internal combustion engines are nowadays predominant as vehicle drive systems. The very high energy density of the liquid fuels used, approx. 40,000 kJ/kg or 30,000 kJ/dm3, is a point in favour of internal combustion engines, especially as the fuel may be carried in simple unpressurised tanks. Using these fuels, vehicles may easily achieve a desired range of 500 to even 1,000 km without refuelling. Refuelling is simple and possible within a very short time. A further advantage of the internal combustion engine is the great speed range that is usable and the simple adaptation to different loads by using the throttle in petrol engines and by changing the injection quantities in diesel engines. In the evaluation of internal combustion engines, the degree of reliability that has been achieved, as well as the availability and the low weight/power relationship must certainly be taken into account. The internal combustion engines' unfavourable torque curve for powering vehicles, especially in the case of high-power low-volume engines, may be adapted to the demands of driving by means of simply-operated manual or automatic gearboxes. The use of large-volume engines with a lower output per litre in vehicle drives would improve the degrees of efficiency and consumptions achievable when driving, but due to the general taxation on engine capacity, this cannot be put into practice. This false premise in the taxation system has promoted the development and the use of lowvolume engines with a high output per litre. Electric drives in vehicles have considerable advantages compared with internal combustion engines, but these are offset by grave disadvantages, so that at present they are only employed in special cases. The greatest advantage is without doubt the fact that the operation of electric motors does not produce any exhaust emissions. They achieve degrees of efficiency of η > 90%, and thus hardly require any cooling; they operate practically silently, have

Research

no fixed installation position due to their design and are very reliable. When the vehicle is stopped, no energy is consumed. Electrically-powered vehicles may be braked regeneratively, during which a major part of the kinetic energy is recovered. The conventional mechanical brakes are, however, indispensable, since they achieve a very high braking power, which is necessary in sudden braking manoeuvres, and can also stop a vehicle when travelling at low speed. Compared to internal combustion engines, electric motors are lighter, simpler and are practically wear-free. Due to their torque characteristics, no gearboxes are required, and when individual drives are employed, distribution gearboxes are rendered superfluous. Due to their characteristics, electric drives are ideal for motor vehicles. The disadvantage in their use is the insufficient energy density of the batteries necessary for the storage of the electric power. In addition, batteries are expensive, have only a limited service life and require a long time for charging. Even the improvements now achieved through great efforts are insufficient to a great extent. Lead-acid batteries achieve an energy density of 110 kJ/kg, and the recently-developed lithium-ion batteries also achieve only 440 kJ/kg. Even considering the losses in efficiency caused by the use of liquid-fuel engines, the energy density of modern batteries is still worse by a factor of 10. Considering the charging capacity, the cost of lithium-ion batteries is around 140 DM/kJ. In comparison to this, we may neglect the cost of tanks for liquid fuels. Vehicles equipped with electric drives thus achieve only low ranges and require long charging times. The power connection required is only available when there is a garage for the vehicle. Electric vehicles will remain, in future, too, an alternative to vehicles powered by internal combustion engines only in very few exceptional cases. There are no exhaust gases produced by the operation of an electric motor, so that areas of high population will have their pollution considerably reduced by the use of such vehicles. The electric power is generated principally by fossil fuels, whose energy is released by burning. The waste gases produced in this process are cleaned very well and transported to higher layers of the atmosphere. The CO2 pollution of the envi-

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Hybrid Drive

ronment is still present, but moved to less sensitive areas. In this context, it is necessary to consider the effective degree of efficiency that is achieved by operating an electrically-powered vehicle. The electrically-powered vehicle itself achieves good degrees of efficiency, which are estimated at η = 70–80%. Losses take place principally in charging and discharging the battery, in the operation of the motor and due to the discharging processes in the battery itself. The degrees of efficiency occurring in the production of the electric power must be included in the balance when we consider electric drives. Although modern gas and steam power stations achieve degrees of efficiency of ηe > 50% and new coal-fired power stations have efficiencies of around ηe > 40%, it may be expected that, altogether, and taking into consideration the older power stations still operating and the losses due to transmission of the electric power from the socket, a degree of efficiency of η = 35% will be available. Electrically-powered vehicles thus achieve a degree of efficiency of ηe ≈ 25%. They are clearly superior to internal combustion engine vehicles in this respect.

5

Operating Range

The comments on the drive concepts internal combustion engine and electric motor show that both types of drive possess advantages and disadvantages. At present, electric drives are employed only in special cases due to their limited range. A combination of both drive systems is possible and very sensible.

curve. The shifting of the operating point caused by this leads to considerably better degrees of efficiency for the engine, and the battery is charged at the same time. A control device can decide which operating mode is used and which load is additionally applied to the internal combustion engine. This control unit would make the decision based on the engine performance characteristics, the driving profile and the state of charge of the battery. Set against this, there are the additional costs of using these hybrid drives; in series production, they are estimated as being at least DM 10,000. In future, hybrid drives will have to gain greater importance with regard to reducing the burden on the environment and preserving resources. The targets for reducing CO2 emissions cannot be achieved in the foreseeable future by other technologies.

6

Conclusions

When the costs are considered, a breakthrough in the use of hybrid drives can only be initiated by legislation. Incentives for the purchase of these systems and limitations on vehicles with internal combustion engines in areas of high population are a suitable means. Such limitations cannot be put into practice at short notice, but should be planned for the medium term. Vehicles which can at a later date be re-equipped as hybrid vehicles can be developed for the transition period. When making these decisions, it should also be remembered that vehicles have an average useful service life of more than 10 years.

In urban traffic, hybrid vehicles can be operated as electrically-powered vehicles. Ranges of 30–40 km ought to be sufficient for pure electric operation, since the conventional drive is still available for emergencies. In this way, the battery capacity requirements remain within acceptable limits. The possibility of regenerative braking for recovering a great proportion of the kinetic energy must be exploited. In the case of slow travelling, stop-and-go traffic and short trips, it also makes sense to employ the electric drive. When the internal combustion engine is operating, the electric motor can operate as a generator and acts as an additional load that is superimposed on the resistance

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