GB2253655A

GB2253655A - Lean burn i.c.engine

Info

Publication number: GB2253655A
Application number: GB9105046A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1991-03-09
Filing date: 1991-03-09
Publication date: 1992-09-16
Also published as: WO1992015778A1; GB9105046D0

Abstract

An internal combustion engine (10) calibrated to operate in a lean burn mode comprising means for supplementing the total output torque, in at least part of the speed/load range, without enriching the mixture beyond the lower limit of lean burn. The source of additional torque may be a supercharger (16), an auxiliary motor (20) or a regenerative braking system (18).

Description

Title INTERNAL COMBUSTION ENGINE Field of the invention The invention relates to a lean burn spark ignition internal combustion engine for a motor vehicle. The term "lean burn" as used herein is intended to signify an air to fuel equivalence ratio in excess 1.3, that is to say at least 30% more air than would be present in a stoichiometric mixture, the latter having aXair to fuel equivalence ratio of 1.0.

The invention is described below by reference to a petrol engine and for such engine an air to fuel ratio of 20:1 is usually taken to be the lower limit of lean burn.

However, the invention can be applied to other fuels, for example alcohol or propane, and in each case the mass ratio of fuel to air will vary with the gram molecular weight of the fuel. For this reason, it is preferable to consider an equivalence ratio which takes the gram molecular weight into account and has the value of 1.0 at stoichiometry (i.e. when the mixture is chemically correct for the components to react fully with one another without any surplus remaining of fuel or air).

Background of the invention An engine which operates in lean burn mode in at least part of its speed/load range is obviously desirable from the point of view of reducing fuel consumption and various advances have been made in engine technology to extend the lean limit at which engine can continue to fire regularly.

However, there are certain disadvantages associated with lean burn which are incompatible with the performance requirements of engines fitted to motor vehicles.

One important problem is that when operating with a weak mixture, the engine lacks power. Therefore, whenever the engine is placed under load it has been necessary to decrease the air to fuel ratio below 20 to 1 in the interest of drivability. Statutory drive cycles used in the motor industry for evaluation of various aspects of engine performance, especially exhaust gas emissions, all include acceleration phases in which the mixture strength would conventionally have needed to be increased outside the range of lean burn.

A further problem is the NOX content of the exhaust gas emissions. In a conventional engine operating with a stoichiometric ratio, the NOx emissions are high but can be reduced to acceptable levels by means of a three way catalytic converter in the exhaust system. With a lean burn engine the NOx content is significantly lower and could comply with statutory regulations were it not for the fact that during much of the drive cycle the engine cannot operate with lean burn. In this case, to further exacerbate the situation, existing catalytic converters cannot be used with a lean burn engine as they require a reducing atmosphere to remove the NOx content and during lean burn the exhaust contains excess oxygen as the fuel in the charge does not use up all the available oxygen.

Ironically, therefore, even though it produces significantly less NOx than an engine operating with a stoichiometric ratio, a vehicle having known lean burn engines following a statutory drive cycle cannot meet the NOx levels laid down in proposed legislation.

Obiect of the invention The invention therefore seeks to provide a lean burn spark ignition internal combustion engine which can comply with statutory regulations on NOx emission and in which performance and drivability are not seriously impaired.

SummarY of the invention According to the present invention, there is provided an internal combustion engine calibrated to operate in a lean burn mode and comprising means for supplementing the total output torque, in at least part of the speed / load range, without enriching the mixture beyond the lower limit of lean burn.

In one embodiment of the invention, the means for supplementing the total output torque include means for increasing the air mass trapped in the cylinders as compared with the mass of air trapped in a naturally aspirated engine. A turbocharger or supercharger may be used for this purpose.

In a second embodiment of the invention, the means for supplementing the total output torque include an electric motor driven by a battery which is charged by the engine during idling and cruising conditions.

In a further embodiment, the means for supplementing the total output torque include an inertial flywheel which is accelerated by the vehicle drive train during braking of the vehicle and/or by its own electric motor.

Of course, more than one source of supplementary torque may be used, for example regenerative braking may be used in conjunction with a supercharger.

These various supplementary sources of torque are known per se but in the prior art they have been used to augment the maximum power output from the engine. Thus, if used with a lean burn engine, they would be brought in during or after fuel enrichment has taken place, that is to say after the mixture strength has been increased beyond the limit of lean burn, in order to maximise the engine's torque output.

By contrast, in the present invention, the supplementary torque is not used to augment the maximum power output of the engine but to enable more torque to be achieved while continuing to run with a weak mixture strength. Thus, the supplementary source of torque in the invention is used as a substitute for enrichment of the mixture strength not an adjunct to it.

The present invention, by enabling the engine to supply the torque required during acceleration phases without resorting to mixture enrichment, allows a lean burn engine to comply with the maximum limit specified for total NOx emissions during the statutory drive cycle, without having to resort to a catalytic converter, which, as earlier stated, cannot currently function properly within the oxidising atmosphere present in the exhaust system of a lean burn engine.

For the purpose of complying with statutory limits on emissions, it is not essential that the engine be operated at all times in lean burn mode. For example, it is possible when idling and during warm up to enrich the mixture beyond the limit of lean burn in the interest of stability. During deceleration (overrunning), the mixture strength may also of itself rise above the lean burn limit. However, at such times, NOX emissions do not pose a serious problem. The invention is essentially concerned with reducing the NOx emissions of a lean burn engine during the acceleration phases specified in statutory cycles by retaining lean burn during these phases and augmenting the engine output by resorting to the supplementary source of torque.

It should also be noted that statutory drive cycles do not extend to times when the engine is being driven for maximum performance, i.e. high acceleration and high speed. At such times also the mixture can be enriched beyond lean burn limit without the engine failing to comply with statutory regulations. Thus the maximum engine performance need not be impaired by meeting these regulations and maximum performance of the engine would be the same as for any other engine of the same capacity.

Indeed, the supplementary source of torque may be used during high performance driving to further enhance the engine output.

Because the construction of lean burn engines, superchargers, turbochargers and electro-magnetic and regenerative braking systems are all known from the prior art, it is believed that the construction and operation of an engine of the invention will be clear to those skilled in the art without the need for further explanation.

Claims

1. An internal combustion engine calibrated to operate in a lean burn mode and comprising means for supplementing the total output torque, in at least part of the speed / load range, without enriching the mixture beyond the lower limit of lean burn.

2. An engine as claimed in claim 1, wherein the means for supplementing the total output torque include means for increasing the air mass trapped in the cylinders as compared with the mass of air trapped in a naturally aspirated engine.

3. An engine as claimed in claim 2, wherein the means for supplementing the total output torque comprise a turbocharger or supercharger.

4. An engine as claimed in any preceding claim, wherein the means for supplementing the total output torque include an electric motor driven by a battery which is charged by the engine during idling and cruising conditions.

5. An engine as claimed in any preceding claim, wherein the means for supplementing the total output torque include an inertial flywheel which is accelerated by the vehicle drive train during braking of the vehicle and/or by its own electric motor.