An off-highway vehicle comprising an engine system, the engine system, comprising: a. a diesel engine configured to drive a driveline of the vehicle; b. an after-treatment arrangement configured to reduce emissions from the engine system; c. an after-treatment heating element configured to raise an operating temperature of the after-treatment arrangement; d. an electric energy storage device; e. a controller configured to direct energy from the electric energy storage device to the after-treatment heating element in order to raise the operating temperature of the after-treatment arrangement; and f. a motor generator configured to drive the driveline of the vehicle, such that the engine system is configured to drive the driveline of the vehicle via the motor generator in an electric mode, via the diesel engine in an engine mode, or a combination of both modes; wherein the system is configured to determine a load demand on the system, and wherein: when the system is running in electric mode, if the demand is determined to be below a lower load threshold, then the controller directs the system to continue to drive the driveline in the electric mode; when the system is running in electric mode, if the demand is above the lower load threshold, the controller directs the system to switch from the electric mode to the engine mode; and if the demand is above an upper load threshold, the controller directs the system to run in both the electric mode and the engine mode, wherein the system is configured to determine an operating temperature of the after-treatment system and, when the system is operating in the engine mode, when the operating temperature is determined to be decreasing and/or when the operating temperature is determined to have reached a pre-determined minimum temperature, the controller is configured to direct the engine to also provide mechanical energy to the motor generator, for conversion to electrical energy and storage in the electric energy storage device. a. a diesel engine configured to drive a driveline of the vehicle; b. an after-treatment arrangement configured to reduce emissions from the engine system; c. an after-treatment heating element configured to raise an operating temperature of the after-treatment arrangement; d. an electric energy storage device; e. a controller configured to direct energy from the electric energy storage device to the after-treatment heating element in order to raise the operating temperature of the after-treatment arrangement; and f. a motor generator configured to drive the driveline of the vehicle, such that the engine system is configured to drive the driveline of the vehicle via the motor generator in an electric mode, via the diesel engine in an engine mode, or a combination of both modes; when the system is running in electric mode, if the demand is determined to be below a lower load threshold, then the controller directs the system to continue to drive the driveline in the electric mode; when the system is running in electric mode, if the demand is above the lower load threshold, the controller directs the system to switch from the electric mode to the engine mode; and if the demand is above an upper load threshold, the controller directs the system to run in both the electric mode and the engine mode, wherein the system is configured to determine an operating temperature of the after-treatment system and, when the system is operating in the engine mode, when the operating temperature is determined to be decreasing and/or when the operating temperature is determined to have reached a pre-determined minimum temperature, the controller is configured to direct the engine to also provide mechanical energy to the motor generator, for conversion to electrical energy and storage in the electric energy storage device.