The McLaren P1™ needed a powerful yet highly fuel-efficient powertrain that offered superb throttle response. “These were essential qualities for the world’s best driver’s car,” says Dan Parry-Williams.
High levels of power were crucial for the car to deliver the performance requirement: to be faster, around a racing circuit, than any other production road car. Sharp throttle response is a key requirement of any great driver’s car. Fuel efficiency has always been a McLaren mantra – great efficiency is the hallmark of quality engineering.
Engine design of a McLaren P1. Copyright McLaren Automotive. All rights reserved.
‘The best all-round solution, we quickly deduced, would be a petrol-electric engine,’ says Parry-Williams. ‘It gives us very high levels of power, instant torque and terrific fuel and CO2 figures. The result is a powertrain that feels like a very strong, normally aspirated engine.’
The twin-turbo petrol V8 and single electric motor – both mounted behind the cockpit in a mid-engine position – have a combined output of 900 hp and 900Nm, with emissions of less than 200g/km. Power is driven to the rear wheels through a seven-speed dual clutch gearbox.
The car can be driven in a variety of modes, powered solely by the electric motor, or using a combination of the two.
Maximum power comes when using both engines together, but even in E-mode the performance is strong. ‘It’s pretty cool to turn up, silently, in a million dollar McLaren,’ notes Paul Mackenzie, Project Director on the McLaren P1™.
In IPAS (Instant Power Assist System) mode, the battery is recharged using surplus energy from the petrol engine – when decelerating, for example. It can also be plugged in to recharge the battery.
IPAS, developed by McLaren, can provide up to 179PS (176 bhp) from the electric motor, and can be deployed at the touch of the steering wheel-mounted button.
Electric motor and instantaneous torque from IPAS
The single electric motor produces 179PS (176 bhp) and 130Nm of torque – although, as it’s geared by a multiple of two, it has an effective torque output of 260Nm. It has been developed by the McLaren Electronics arm of the Group, and is unique to the McLaren P1™. Like all electric motors, it can produce maximum torque instantly, greatly increasing the throttle response of the McLaren P1™.
“It helps give extra torque anytime, anywhere, and comes in instantly,” notes Chief Test Driver Chris Goodwin. “It makes a huge difference and fills in the holes in the torque curve that you often get with turbo engines.”
As the electric motor sharpens throttle response, so bigger turbochargers – which traditionally inhibit throttle response – can be used to boost overall power. The electric motor and 3.8-litre twin-turbo V8 petrol engine thus work seamlessly together, serving up both instant throttle response and very high maximum power.
The electric motor has internal rotor cooling – unusual for an automotive electric motor – which enables the engine to produce maximum performance for longer periods, and an outer jacket is also used to further aid cooling of the electric motor.
The electric motor on the McLaren P1™ weighs 26kg, and produces more than double the power of the KERS unit used in Formula 1 (179PS versus 82PS). The additional power from the electric motor can be deployed through the IPAS button mounted on the steering wheel.
Battery
The ground-breaking, lightweight battery pack used in the McLaren P1™, offers greater continuous power density than any other car battery pack on sale today. McLaren has prioritised power delivery over energy storage, and so the system is designed to deliver power rapidly for high performance acceleration. The battery is capable of providing instant additional power, which is accessible through pressing the steering wheel-mounted IPAS button. This set-up can provide up to 179PS (176 bhp) and an electric range of over 10 kilometers on the combined European drive cycle.
McLaren P1 Doors open. Copyright McLaren Automotive. All rights reserved.
Energy, which would normally be wasted, is captured by the electric motor when lifting off the throttle, and harvested in the battery, especially in higher gears. The decision was taken to maintain a consistent feel during braking – crucial for performance driving – and for this reason, direct braking kinetic energy regeneration is not employed. ‘The priority is the driving experience,’ notes Dan Parry-Williams.
In addition to the battery being charged via the engine, the McLaren P1™ is also equipped with a plug-in charger which can recharge the battery, from empty, in as little as two hours. The plug-in charger can be stored in the luggage compartment, although the customer may choose to store it off-board – in a garage or the pits – to save weight.
A fast ‘pit lane charging’ function means the battery can be ‘quick charged’ in only 10 minutes, if needed, which provides a complete, 100 per cent charge. The ‘Charge’ button on the fascia allows the driver to recharge the battery quickly, using the V8 engine as a generator, in preparation for electric-only use (to extend range) or for a hot lap (where maximum additional electric power can be utilized).
The high power density has been achieved through a combination of ultra high power cells, low pack weight and an innovative cooling system. The battery weighs just 96kg, and this is crucial in optimizing the performance of the McLaren P1™. It is mounted between the cabin and engine bay for best weight distribution, within the high-strength Formula 1-grade carbon fiber MonoCage chassis. This seals the unit in the vehicle, thus avoiding the added weight of any unnecessary battery packaging.
The battery itself uses six modules, each of 54 cells (324 cells in total), and uses a Battery Management System (BMS) with active cell balancing, which is able to transfer charge from cell to cell to maintain accurate balance throughout the power pack, thus ensuring optimum performance and durability. Due to the amount of power being supplied by the battery, complex cooling is required to guarantee cell performance and reliability. The coolant flow is balanced to ensure each cell is cooled to the same temperature across the entire pack. Two extremely accurate and fast safety-critical monitoring boards are fitted to each cell module, reporting on battery cooling, state of charge and battery health.
Transmission
The electric motor is integrated into the 3.8-litre twin-turbo V8 petrol engine, and drives a dual-clutch seven-speed gearbox. All drive – either in electric or IPAS mode – goes through the gearbox. As the electric motor can be decoupled from the petrol motor, so the car has, in effect, three clutches.
Steering wheel of a McLaren P1. Copyright McLaren Automotive. All rights reserved.
The gearbox is an uprated version of the unit found in the 12C, but additional cooling is required to manage the more powerful IPAS petrol-electric powertrain. Two air-blast clutch coolers are also fitted to optimize oil cooling.
Manual gear-shifting is by paddles mounted on a rocker behind the steering wheel: to upshift pull with the right fingers or push with the left, and vice versa to downshift. The paddles are made from carbon fiber and optimized for weight-saving. They have also been ergonomically designed to allow ease of use whilst also operating the IPAS and DRS buttons.
The default for the McLaren P1™ is a fully-automatic mode, which is especially useful for city driving. The paddles can also be used while in automatic mode, and a fully manual gearchange can be selected via a button on the Active panel. In E-mode, the McLaren P1™ always drives in automatic.
E-mode
The E-mode is the most economical mode available with zero tailpipe emissions and almost silent running. Where possible, the car will drive in electric-only power, but should there be insufficient battery power, the petrol engine will automatically start. The car is also eligible to drive in towns or cities that may restrict or ban internal combustion engine vehicles.
McLaren P1 photo by Patrick Gosling. Copyright McLaren Automotive. All rights reserved.
In E-mode, the McLaren P1™ can travel more than 10km – enough for most city journeys – and at speeds of over 160km/h. When the battery is empty, the petrol engine will automatically start to maintain drive and charge the battery. Performance remains the same as in electric-only drive. This is achieved through an ECU map restricting performance to the equivalent of the electric motor.
E-mode is selected before setting off via the E-mode switch on the dashboard prior to switching on the ignition. If already on the move, the powertrain will then instantly change to electric power when the E-mode button is pressed.
By pushing the Charge button (next to the E-mode button), the petrol engine will quickly recharge the battery. When the battery is fully recharged, in E-mode the petrol engine will stop automatically. Recharging the battery in this way takes only 10 minutes.
In E-mode, the twin clutch seven-speed gearbox will change gear automatically. Operating the paddles will have no effect. The handling settings are the same as the Normal suspension mode. It is not possible to select other handling settings.
IPAS petrol-electric mode
The default mode for the McLaren P1™ is in IPAS drive, when both petrol and electric motors combine. Together, combined power is 916PS (903 bhp) and torque is 900Nm – although this is limited to protect the clutch.
McLaren P1 Winter Testing in Arjeplog Sweden 2013. Copyright McLaren Automotive. All rights reserved.
The electric motor does far more than just add extra ultimate power and torque. The instant response of the electric motor provides sharper throttle response that is normally associated with a normally aspirated motor. This is especially beneficial when mated to a petrol engine using large turbochargers. ‘It’s particularly useful just after gear shifts to “fill in” the torque gap when the turbos are responding,’ says Chief Test Driver Chris Goodwin. A further benefit is that the electric motor can provide faster upshifts. This is achieved by the electric motor providing negative torque, which makes the engine revs drop as quickly and efficiently as possible to the required engine speed for the upshift.
The use of an electric motor, and an all new pressure charging system, enables the McLaren P1™ both to have sharper throttle response and more top-end power – the perfect combination for high performance.
Unlike many other powertrains that use both petrol and electric power, the petrol motor on the McLaren P1™ is always active – unless it’s in E-mode. It will not continuously cycle between drive modes. “We didn’t even try to do it,” says Dan Parry-Williams. “We felt that it would detract from the driving experience to have more unexpected engine starts than absolutely necessary. Believe me, you’d notice if a 737PS petrol engine suddenly starts up behind you.” At standstill, such as at traffic lights or a junction, the petrol engine will switch off automatically when the brake pedal is depressed, and restart again when released.
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