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A burning issue

24 May 2011 2 Comments

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The regular re-fuelling of our cars nudges many people’s ‘green’ conscience, reminding us of the energy we consume simply going about our daily business. Not to mention the financial cost of what flows from the pump.

Whether to conserve world resources, minimise climate change or reduce the cost per mile, fuel efficiency is a big issue and one that the automotive industry continues to address.

Injecting some science

At the heart of the engine lies the combustion chamber where atomised fuel burns to produce energy. Today direct fuel injectors are the norm. These highly engineered units release diesel or petrol into the combustion chamber so that the fuel vaporises and mixes with oxygen to form a flammable mixture.

The size of droplet that a fuel injector produces influences both the speed with which fuel atomises and the mixing efficiency in the combustion chamber. By controlling these factors car manufacturers deliver the power output we demand while at the same time minimising emissions and maximising fuel efficiency.

A tough analytical ask

Of course, you need to be able to measure the droplet size before you can control it and this is not always an easy task. Atomisation is such a rapid event that gathering data fast enough to get a clear picture of what is going on is challenging. Also these sprays are highly concentrated and fine enough to make instrument contamination a major concern. These factors make for a tough analytical challenge but laser diffraction has proved its worth on all counts.

The data below were generated using Spraytec, a specialist spray measurement system. These finely resolved concentration and droplet size data indicate in detail the dynamics of droplet formation, during the atomisation event.

This profile is typical for atomisation by applied pressure, though a nozzle. As the valve opens at the beginning of the spray event, concentration builds rapidly and droplet size quickly falls. Flow through the valve then reduces from this peak value, droplet size effectively plateauing at a relatively fine size.

Targeting leaner, greener transportation

A number of parameters can be varied to improve performance – the geometry of the nozzle, for example, and the pressure applied upstream during atomisation. The aim is to atomise as much of the fuel as possible to the optimal droplet size; minimising the duration of the initial phase is one important strategy.

Data of the type shown above help guide engineers and are of great value in quantifying the effect of different parameters during development.

If you’d like to read a little more there’s an application note I can recommend on our website:  High resolution characterisation of fuel injection systems

If you work on sprays…

Can you share any experiences of the challenges of spray characterization in your field, especially when it comes to, measuring droplet or particle size?