LEAN-BURN ENGINES

Lean Burning of Fuel/Air Mixture in Internal Combustion Engines



Lean Burning of Fuel/Air Mixture in Internal Combustion Engines

Introduction

An internal combustion engine burns the fuel inside the engine. Cars, motorbikes and several generators have this engine. Gases are produced for the working of engine by the combustion of the fuel (www.bbc.co.uk).

Lean is the fuel/air mixture with less amount of fuel as needed for consuming all the oxygen present in the air. Lean burning occurs by allowing the same amount of air into the cylinder as for maximum power, and reducing the amount of fuel while closing the throttle valve or expelling the fuel/air mixture. This approach is beneficial in reducing the pumping loss in engines, which occurs by the energy consumption resulting in the reduction of fuel efficiency due to the creation of partial vacuum while drawing air into the cylinder (prius.ecrostech.com).

Discussion

Lean burning also causes damage risks of pre-ignition, detonation, and respiratory problems due to the nitrogen oxides emissions. Its efficiency and reliability varies with variant gases used as fuel in the internal combustion engine which is achievable with a number of techniques. Moreover, depending on the lean level of the fuel/air mixture, the two fundamental reasons hinder the conventional engines to operate in lean fuel/air mixture;

1.The engine fails to combust if the mixture will be too lean.

2.Certainly reduced fuel concentration leads to lower output.

(www.autozine.org).

However, Lean-burn engines evade these issues by taking on a highly dexterous mixing method. The use of special shaped pistons, together with tubed manifolds according to the piston produces a swirl within the combustion chamber which mixes the fuel and air completely. Thus, the improper mixing of fuel particles minimizes which allows complete burning. It decreases the pollutant level with lowering the air/fuel ratio from 14:1 to 25:1 with no compromise on output. Nowadays, Lean Burn technology has greatly progressed into Direct Injection as Toyota, Mitsubishi and Nissan all have concentrated in DI engines development (www.autozine.org).

Direct Injection Petrol engine - Mitsubishi GDI

Mitsubishi has practised Gasoline Direct Injection (GDI) in various engines (from 1.5-litre four to 4.5-litre V8.) thus its major productions are GDI-equipped. Its engines consume 20-35% reduced fuel, produce 20% reduced Carbon dioxide emission and generate 10% extra power as compared to the conventional engines (www.autozine.org).

Theory

GDI technology works on the Lean-burn technology with the difference in the application of directly fuel injection system. GDI has two main benefits over conventional engines,

The fuel enters directly into the combustion chamber at high pressure, just before the spark plug ignition. This approach minimizes the charge stratification for igniting the ultra-lean fuel/air mixture.

The need of throttle is also replaced by GDI, which eliminates the pumping loss.

(www.autozine.org)

In conventional engines, the fuel injection goes through the intake port first before getting into the combustion chambers. This cannot occur directly as the fuel will not be spread uniformly. While GDI assures the mixing of all air and fuel at the same rate (www.autozine.org). This can be represented as,

Figure 1 (www.autozine.org).

GDI has upright straight intake port with a concave section piston ...