This paper aims to characterize the engine start activity profiles and emission potential of various plug-in hybrid electric vehicle (PHEV) models by examining the characteristics associated with engine starts, identifying the travel conditions that trigger engine starts, and determining the frequency of different types of engine starts. The study analyzed on-road vehicle data from six PHEV models: Toyota Prius Plug-in, Ford C-Max Energi, Ford C-Max Fusion, Toyota Prius Prime, Chrysler Pacifica, and Chevrolet Volt. An analysis on travel conditions before engine starts revealed that low state-of-charge is the dominant engine start trigger for PHEVs with high all-electric range whereas high vehicle power requirement is the most critical trigger for PHEVs with low all-electric range. For PHEVs with mid-range capabilities, several vehicle specifications, ranging from peak electric motor power to curb weight, could be engine start determinants. A strong inverse correlation exists between battery capacity and the annual frequency of engine starts but this relationship does not hold for cold and high-power cold starts. Both the low and the high battery capacity PHEVs logged fewer cold starts than the mid-sized battery vehicles, indicating that there could be a fundamental tradeoff between engine start emissions and fuel displacement for PHEVs to a certain degree. Despite this tradeoff, all PHEV models in the study logged fewer cold starts than comparable conventional internal combustion engine vehicles, performing the same trips. Ultimately, long-range PHEVs with high battery capacity are found to be ideal for both curbing start emissions and reducing fuel use.

Numerous studies have found that engine starts at suboptimal travel conditions, such as low engine block and catalytic converter temperatures, are major sources of harmful environmental air pollutants in conventional internal combustion engine (ICE) vehicles. These emission-heavy engine starts are typically referred to as cold starts, given their direct link to low engine operation temperatures. The engine start emissions of plug-in hybrid electric vehicles (PHEVs) can differ dramatically from the emissions of traditional ICE vehicles. Unlike ICE vehicles, PHEVs can use energy from a battery, an ICE, or a combination of the two to attain propulsion power, enabling them to invoke the engine at any moment within a specific trip. PHEVs can even potentially finish a trip or a travel day without a single engine start, emitting zero tailpipe emissions. When a PHEV’s power requirement exceeds the power that can be provided by its all-electric propulsion system, it can invoke the ICE to assist the vehicle to meet this requirement. PHEV engine cold starts that occur during a trip (not at the start) as a result of high-power requirements have been shown to emit even more pollutants than regular cold starts, given the engine must quickly rev itself up to support the vehicle’s high operating speed and torque. Cold starts and high-power cold starts can negate the environmental benefits of PHEVs that are designed to reduce fuel use by maximizing the dwell time or “engine cooling period” between engine starts.

Given there is very little empirical research on the tailpipe emissions associated with PHEVs, this study seeks to characterize the engine start activity profiles and emission potential of various PHEV models by defining the characteristics associated with engine starts, identifying the travel conditions that trigger engine starts, and determining the frequency of different types of starts. This study examines on-road vehicle data from six PHEV models: Toyota Prius Plug-in, Ford C-Max Energi, Ford C-Max Fusion, Toyota Prius Prime, Chrysler Pacifica, and Chevrolet Volt. These models account for around 67% of PHEV sales in California to this date and cover a wide range of vehicle specifications; this allows for the comprehensive analysis of the impact of these specifications, including battery capacity and drivetrain configuration, on the engine start emission potential of PHEVs.