Altitude compensation for internal combustion engine

Abstract

In order to address turbo lag at altitude, a vehicle boosts output torque of an internal combustion engine with electric motor torque generated from a battery. The residual charge of the battery is increased at altitude to provide a sufficient reserve for the corresponding increase in turbo lag. The invention is typically applied to a parallel hybrid vehicle.

Claims

1. A method of defining a residual capacity of a battery of a vehicle having an internal combustion engine and a forced induction device, said battery being adapted to supplement output torque of said engine via a rotary electrical machine during a lag period of the forced induction device, said method comprising: defining a minimum capacity of said battery; defining a residual capacity above said defined minimum capacity, said defined residual capacity increasing with vehicle altitude; driving the vehicle in a first mode of operation when a capacity of the battery is above the defined residual capacity, using only the rotary electrical machine to provide output torque; and driving the vehicle in a second mode of operation when the capacity of the battery is below the defined residual capacity and above the defined minimum capacity, using the rotary electrical machine only to provide supplemental torque to said engine only during the lag period.

2. The method of claim 1, wherein said defined residual capacity is substantially zero at sea level.

3. The method of claim 1, wherein said defined residual capacity increases in proportion to the increase in vehicle altitude above sea level.

4. The method of claim 3, wherein the increase in said defined residual capacity is directly proportional to the increase in vehicle altitude.

5. The method of claim 1, and including the step of charging said battery from said vehicle to maintain battery charge above said defined residual capacity.

6. The method of claim 5, wherein charging comprises generating electrical power from a generator of the vehicle engine and/or by regenerative braking of the vehicle.

7. The method of claim 1, wherein said defined residual capacity is periodically calculated according to the instant altitude of said vehicle.

8. The method of claim 1, wherein said battery is a traction battery adapted for driving said vehicle independently to the internal combustion engine.

9. The method of claim 1, wherein said battery is associated with a belt integrated starter generator of a vehicle.

10. The method of claim 1, wherein said forced induction device is a turbo.

11. The method of claim 1, wherein said forced induction device is a supercharger.

12. The method of claim 1, further comprising: sensing that engine output torque is lower than the torque demanded by the driver of the vehicle during the lag period of the forced induction device; and supplementing engine torque with torque from the electric machine to provide a combined torque substantially equal to the torque demanded by the driver during the lag period.

13. The method of claim 12, wherein said forced induction device is a turbo or a supercharger.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a graphical representation of demanded and actual vehicle torque of an internal combustion engine at sea level;

(3) FIG. 2 is a graphical representation of demanded and actual vehicle torque of a internal combustion engine at a significant altitude;

(4) FIG. 3 is a graphical representation of the variation with altitude of residual battery capacity according to an embodiment of the invention; and

(5) FIG. 4 is a graphical representation of an increase in state of battery charge with altitude, according to an embodiment of the invention.

DETAILED DESCRIPTION

(6) With reference to FIG. 1, torque demand of a vehicle driver is represented by the solid line 11, and corresponds to depression of an accelerator pedal from minimum to maximum at sea level.

(7) Torque response of a turbocharged engine is represented by the dotted line 12, and it can be seen that the rise in engine torque is more or less immediate until t.sub.1. This rise represents natural aspiration of the engine.

(8) From t.sub.1 to t.sub.2, the torque response of the engine is delayed due to the turbo lag effect, and from t.sub.2 to t.sub.3 the torque response overshoots marginally until settling at a level corresponding to steady driver demand.

(9) The effect of turbo lag is somewhat exaggerated in FIG. 1 for the purposes of illustration, and in practice the turbocharger may be designed to minimize the turbo lag effect at sea level such that it is not noticed by the vehicle driver.

(10) FIG. 2 represents torque response of the same engine at a significant altitude, for example 2000 m above sea level. In this case natural aspiration of the engine allows the torque response to follow demand for a more limited period, because air pressure is reduced. Thus the turbo lag effect commences at t.sub.4 and continues to t.sub.5, with a slight overshoot to t.sub.6.

(11) It is apparent that available torque lags demand by a significant period. For example a demand of maximum torque by the driver may result in a delay of 1-2 seconds before the vehicle engine delivers maximum torque. This delay is noticeable, and undesirable. The turbo is a type of forced induction device. Superchargers are also forced induction devices. Supercharged engines may also be susceptible to lag at altitudes above sea level. However, lag is much less likely in supercharged engines because this type of forced induction device is directly linked to engine speed. Nevertheless, where turbo lag is mentioned in the following text, it is to be understood to include the lag from other forms of forced induction devices.

(12) The invention provides for engine output torque to be boosted to compensate for the turbo lag effect, whilst ensuring that sufficient reserve of electrical energy is available notwithstanding that the vehicle is normally operated as a parallel hybrid with electric traction used in preference to the internal combustion engine.

(13) With reference to FIG. 3, the state of charge (C) of a battery of a hybrid vehicle used in sequential parallel hybrid mode is represented. Charge depletes over time from maximum (100%) to a predetermined minimum (e.g. 20%) after which the vehicle relies solely upon the internal combustion engine thereof. Straight line depletion is illustrated for simplicity, though in practice a more complex discharge characteristic may apply.

(14) According to the invention, the state of battery charge is raised above the minimum in the event that the vehicle is used at altitude, so that for example the residual charge is 40% at 2000 m above sea level. Vehicle altitude may be sensed by, for example, an atmospheric pressure sensor, and the electrical output signal from such a sensor may be used to determine a progressive increase in the target residual charge. FIG. 4 shows a straight line relationship whereby residual charge is increased linearly from 20% to 50% as altitude increases from sea level to 3000 m.

(15) In use electric motor torque is used to supplement the torque output of the internal combustion engine in order to substantially eliminate the turbo lag effect at altitude, whilst preserving a minimum state of charge in the battery. Conventional techniques may be used for temporarily or momentarily operating the vehicle in a mode where electric traction assists the internal combustion engine.

(16) The invention allows compensation for the turbo lag effect without special measures for increasing the mass flow of air through the engine at altitude.

(17) Hybrid vehicles typically may allow for re-charging of the traction battery by a generator driven by the internal combustion engine, typically on overrun, or by energy recovery under braking. Accordingly the vehicle may charge or re-charge the traction battery to achieve a desired residual charge greater than the minimum charge. Such an arrangement is particularly desirable where vehicle altitude changes significantly during a driving event. Residual charge may be allowed to fall in the event of a significant reduction in altitude.

(18) The invention may be applied to a vehicle starting from rest, for example when pulling away from a road junction, to give immediate urge on demand at all altitudes. The invention may also be applied to a moving vehicle during a change of gear ratio, in the event that engine speed and hence torque output momentarily reduces.

(19) The example described herein concerns sequential parallel operation of a hybrid vehicle comprising an internal combustion engine and an electric motor/traction battery. It is however also applicable to other kinds of vehicle where momentary electrical power assistance is available, such as a vehicle filled with a belt integrated starter generator (BISG). A BISG vehicle allows an engine to be stopped and automatically restarted, for example during a temporary halt at traffic lights. A BISG system may be used to momentarily supplement engine torque of a running engine so as to ameliorate the turbo lag effect.