How to resolve low-speed pre-ignition

Demand is driving innovation in the areas of reducing carbon dioxide and nitrous oxide emissions and delivering better fuel economy. Consumers and governments want cleaner air and more efficient, environmentally-friendly vehicles.

One of the issues is to ensure more torque, pressure and power without increasing emissions or the amount of fuel used. The issue of low-speed pre-ignition (LSPI) is one that must be resolved to reduce emissions and boost fuel economy.

Smaller engines, bigger output

One initiative that has been taken onboard by manufacturers such as General Motors and Ford is that of reducing - or downsizing - their petrol engines while developing systems to deliver boosted output. Both manufacturers have developed one-litre engines with three cylinders that achieve the same outputs as engines with four cylinders and 1.6-litre capacities. In addition, these Ecotec and EcoBoost engines achieve lower emissions of carbon dioxide and an improvement in fuel economy of around 20 per cent and lower CO2 emissions. They also deliver efficiencies that encompass reduced gasses-to-wall heat transfer, improved torque when the engine is operating at lower speeds, higher ratios in transmission for engine down-speeding, and reduced losses in mechanical pumping.  GM and Ford have both stated that demand for these engines will mean increased production to satisfy markets around the world. Ford is already making more than 100,000 EcoBoost engines each month, with the aim of making this technology available in up to 80 per cent of its fleet, while GM is aiming to produce several million of these engines each year in five different manufacturing plants around the globe. 

Preserving performance

Early models of downsized engines were unfortunately found to have reduced emissions and increased economy at the expense of performance. As a hedge against decreased output, auto manufacturers began to incorporate turbochargers into downsized engines to give operating pressure a boost. This move has been buoyed by the massive advances in TGDI - turbocharged gasoline direct injection - that have taken place during the past five or six years. 

LSPI can be the result of boosting downsized engines and generally happens during low-speed operation when sudden acceleration occurs. LSPI is thought to be the result of particles of oil and fuel that collect in the combustion chamber, ignite and combust. This can lead to engine damage and spikes in the engine’s pressure. 

As damage can be extensive, research into lubricants, engine design and fuel design is needed. As LSPI can be an obstacle to producing vehicles with better performance, this is an issue that needs addressing. Unfortunately, there is no simple solution.

As more engines are downsized, LSPI will become more common. Tackling the problem will need more investigation into optimal fuels, lubricants and engine design whereby all three elements work together. Fuel injection can affect the surface tension of oil coating combustion chambers; therefore, improving oil and fuel design may contribute to a solution. Until this is resolved, manufacturers may be limited in their ability to maximise fuel efficiency and performance.