Mitigating the effects of LSPI with downsized engines

In September 2017, a new vehicle test cycle was introduced for new car models type-approved in the EU and, in September 2018, for all new vehicles sold. The Worldwide Harmonised Light Vehicle Test Procedure (WLTP) has been designed to better represent the carbon dioxide and pollutant emissions generated in modern day driving situations.

The new CO2 targets are that EU countries reduce emissions from new cars by 37.5% by 2030 compared with 2021, while emissions from new vans will have to be 31% lower. There is also an interim target of a 15% cut for cars and vans by 2025. The average CO2 emission performance for new vehicles sold in the EU in 2017 was 31% lower than those sold in 2000.

Pollutant emissions legislation for new vehicles has also become increasingly stringent with limits for nitrogen oxides (NOx) emissions down 84% since 2000, particulate matter (PM) reduced to virtually zero and carbon monoxide (CO) emissions at very low levels. A new on-road measurement has been introduced for both NOx and particle emissions. Real Driving Emissions (RDE) testing measured these pollutants in real traffic on pre-defined routes in order to ensure compliance of vehicles under actual use conditions.

One of the main strategies for CO2 emissions reduction has been improving engine efficiency through engine downsizing. A downsized engine has a smaller displacement, yet operates at higher engine pressures in a more efficient part of the engine map. This efficient operation together with friction reduction from fewer pistons and bearings, realises a resulting improvement in fuel economy.

Research and development of the internal combustion engine (ICE) have allowed engineers to obtain more useful power from a given quantity of fuel. The ICE ignites fuel and oxygen at a fixed mixture ratio, the engine power is thus limited by the amount of air it can breathe. Engine down-sizing reduces swept volume and thus the amount of available air. To recover performance a turbocharger is used to increase intake air density, thus recovering engine power at a higher efficiency.

There are plenty of examples of downsized engines already in the market. Ford’s 1.0-litre EcoBoost turbocharged gasoline engine is available with outputs of 99bhp, 123bhp and 138bhp, meaning it effectively replaces naturally aspirated (non-turbocharged) 1.4, 1.6 and even some 1.8-litre engines from the past.

So, if you look at the Ford example, the Fiesta 1.25-litre non-turbocharged gasoline with 81bhp can manage 54.3mpg and emits 122g/km of CO₂, while it has enough power to accelerate from 0-62mph in 13.3 seconds. The smaller turbocharged engine in the Fiesta 1.0-litre EcoBoost with 99bhp returns 65.7mpg, emits 99g/km of CO₂ and can sprint from 0-62mph in 11.2 seconds.

One of the challenges of introducing smaller engines with higher power density is the risk of LSPI (low speed pre-ignition): a form of abnormal combustion that can occur in small engines with high compression. These downsized, highly-boosted small engines under low speeds and high loads have this very destructive possibility of low-speed pre-ignitions which can be very damaging to an engine.

There are certain manufacturers as well as the American Petroleum institute (API) who now impose a LSPI test. Oil is implicated in triggering LSPI, little droplets of oil that end up in the combustion space can trigger it. There are certain ingredients in oils that make that worse and so Castrol must balance that.

LSPI cause and effects

LSPI is a premature combustion event that can occur before the spark ignition when the new breed of smaller, turbocharged engines operate at low speeds and high loads. LSPI, caused by small droplets of oil and fuel in the combustion chamber that prematurely ignite the entire fuel/air mixture prior to the spark, causing abnormal combustion.

The resulting large uncontrolled pressure rise in the cylinder is audible. Repeated events can cause engine hardware failure, including broken spark plugs and cracked pistons. A single LSPI event is enough to cause severe engine damage.

To prevent LSPI from happening you need to understand the factors that influence the undesired pre-ignition. There are numerous causes of LSPI such as engine design and the composition of fuels and lubricants. As far as lubricants are concerned, one ingredient that is known to impact LSPI is the detergent chemistry, or more importantly the calcium content.

Castrol has been studying this phenomenon closely for several years, both internally and in collaboration with engine manufacturers. This includes detailed scientific analysis of how oil and fuel factors impact LSPI in engines and how to formulate solutions to mitigate it.

By understanding how various elements and additives impact LSPI Castrol has been able to carefully formulate the whole lubricant package in order to address LSPI whilst giving class-leading performance as a lubricant. Engine tests for LSPI are increasingly being added to OEM specification requirements, meaning Castrol must be able to demonstrate LSPI protection in order to gain approvals.

There are other influencing parameters within the lubricant formulation having influence on LSPI, such friction modifiers and anti-wear chemistry. It’s crucial for engineers to understand these parameters and how to balance the lubricant formulation if they want to mitigate LSPI and maintain overall performance.