The challenges of starting and running a car in cold conditions are well-known, but why exactly do they happen? In this blog, we’ll dive in to some of the major reasons why combustion and hybrid powertrains struggle in the cold, and what the industry is doing to improve cold-weather performance.
One challenge engines face at low temperatures is the behavior of fuel within the engine. Depending on whether a vehicle runs on petrol or diesel, the issues it must contend with are different.
Diesel fuels tend to become more viscous in lower temperatures, with waxy crystals forming at below-freezing temperatures. The result is that the flow of fuel into the combustion chamber slows down, starving the engine. To fix this, drivers need to use winter-grade fuel blends that include chemicals that reduce coagulation in the cold.
Along with reduced fuel flow, diesel also suffers from the reduced efficiency of compression ignition. Diesel ignition relies on the heat of the air in an engine cylinder to ignite fuel, which comes from said air being squeezed during the engine’s compression stroke. But if the cylinder and wider engine is cold enough, this heat can rapidly disperse away from the compressed air before ignition can occur – instead warming up the cylinder and engine block. The solution to this usually means pre-heating systems such as glow plugs, minimizing the heat lost during compression strokes.
While gasoline vehicles also become less effective in the cold, there are somewhat different reasons at play.
While part of the loss of efficiency is due to the wider engine leeching heat energy from the combustion chamber to warm up, much of the cold-related problems facing gasoline engines comes down to how temperature changes gasoline’s behavior as a liquid and a vapor. Gasoline combustion relies on the ease of which gasoline evaporates, with fuel evaporation in the engine cylinder creating an evenly distributed air-fuel mixture that produces a complete burn. However, in colder temperatures, gasoline is less prone to evaporating – resulting in a less even burn with greater knocking and vibration, meaning reduced efficiency.
To compensate, modern engines employ a number of advanced technologies to read the engine’s contents and state in real time, and orchestrate a response via engine control unit (ECU) chips. ECUs can read temperature, engine speed, and burn patterns, and can instruct the engine to compensate for changing conditions such as changes to the air-fuel mix or ignition timing. Then there are also high-pressure direct fuel injection techniques, which spray fuel into the engine cylinder as a mist – rather than injecting fuel as a liquid via the intake port.
Starting and running a car in cold conditions poses many challenges for vehicle owners
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The traditional lead-acid battery has been a staple of vehicles for decades. In a pure-combustion car the battery is critical to starting the engine: it’s what powers the starter motor that generates initial spin for the crankshafts, and ensures sparkplugs ignite fuel in the cylinder heads.
In normal conditions these batteries are continually charged and maintained by the engine. However, according to the American Automobile Association, at freezing temperature a battery’s ability to deliver power can be reduced by around 30% owing to a slowdown in the chemical reactions that generate charge. The result is difficulty starting the engine in cold temperatures – either the spark plugs are not igniting the air-fuel mix in the engine cylinders, or there’s no initial momentum to compress the air-fuel mix down due to a lack of crankshaft spin.
In recent years, however, we have also seen the rise of lithium-ion batteries – these are key to powering mild hybrids (MHEVs), full hybrids (HEVs), and plug-in hybrids (PHEVs).
Lithium-ion batteries store more energy than lead-acid batteries on per-kilogram basis, and can fully power a vehicle when sufficiently scaled up. In MHEVs, HEVs, and PHEVs, these batteries serve the role of powering the vehicle via a starter-generator mounted to the engine at low speeds, and in HEVs and PHEVs they also power a standalone traction motor that provides power to the wheels.
Like lead-acid batteries, lithium-ion batteries also become less efficient at providing power at freezing temperatures and below. The result is not just a reduced ability for an engine to start-up, but also a fall a powertrain’s efficiency as the electrical components of the hybrid are unable to assist the engine as designed.
To combat some of these issues, many modern vehicles use advanced 12-volt batteries, such as EFB and AGM batteries, which are better able to handle repeated starting and charging cycles. In hybrids, battery thermal-management technology, including active heating in some models, can help mitigate cold-weather performance issues. The same intelligent technology behind ECUs – real-time sensing and interventions to respond to the cold – is thus key to guaranteeing battery performance in the cold.
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Beyond fuel and battery-related challenges, every element of a powertrain experiences increased stress when exposed to low temperatures. Even with sufficient antifreeze and other precautions, many systems become less effective.
One of the major issues facing the whole powertrain is that oil and lubricant becomes more viscous in the cold. This increases friction and an engine’s mechanical losses. Other factors, such as water contamination, can also become a concern, as ice formation can block fuel lines or damage components.
While cold weather posed a serious challenge for early combustion engines, advances in engine technology and supporting systems allow modern vehicles to continue performing even at very low temperatures. Although most vehicles will experience some reduction in efficiency when temperatures drop below freezing, modern cars are generally capable of delivering a comparable driving experience, even when snow is on the ground. Through allowing vehicles to read and respond to their conditions in real-time, modern powertrains allow for far greater efficiency in cold weather than ever before.
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