Turbocharger operation to increase brake effectiveness

A motor vehicle comprising:

• an internal combustion propulsion engine coupled to road-engaging drive wheels through a drivetrain for propelling the vehicle;
• the propulsion engine comprising engine cylinders within which pistons are reciprocated to propel the vehicle by delivering torque through the drivetrain to the drive wheels when fuel is combusted within the engine cylinders, but when fuel is not being combusted within the engine cylinders and the vehicle is rolling on a road surface underlying the drive wheels, the pistons are reciprocated by the drive wheels acting through the drivetrain;
• the propulsion engine further comprising an intake system, an exhaust system, an intake manifold through which air which has passed through the intake system enters the engine cylinders to support combustion, and an exhaust manifold through which exhaust resulting from combustion leaves the engine cylinders for ensuing passage through the exhaust system;
• a turbocharger comprising a turbine in the exhaust system operated by exhaust from the exhaust manifold and a compressor in the intake system operated by the turbine for creating pressure in the intake manifold exceeding ambient atmospheric pressure;
• an exhaust gas recirculation system for diverting some exhaust from the exhaust system to the intake system to create an air-exhaust mixture entering the intake manifold and for controlling at least one of a) mass of diverted exhaust and b) cooling of diverted exhaust;
• a compression release brake which, when the pistons are being reciprocated by the drive wheels acting through the drivetrain, rather than by in-cylinder combustion, can be activated to dissipate energy of air which the respective piston has compressed within at least one engine cylinder by causing air which the respective piston has compressed to be released into the exhaust manifold so that energy of the released air is not recovered as a contribution to propulsion of the vehicle;
• and a control which, in response to activation of the compression release brake when the vehicle is operating at some elevation above sea level and the compressor is operating in a region of an operating map which is creating boost air in the intake manifold which would cause the compression release brake to decelerate the vehicle more slowly at that elevation than it would at sea level for the same operating conditions of the vehicle and engine other than altitude, causes the compression release brake to decelerate the vehicle less slowly by causing the exhaust gas recirculation system to reduce at least one of a) mass of diverted exhaust and b) cooling of diverted exhaust.