The arrival of new PureTech petrol engines in the Citroën range is further proof of the Brand’s commitment to ‘Créative Technologie’. The new family of three-cylinder engines has equivalent or superior power compared with the previous generation powerplants, combined with significantly lower fuel consumption and greenhouse gas emissions, down by roughly 25%.
Available on several Brand ranges – C3, DS3 and C-Elysée (for International markets) – the new engine family is produced at the Trémery site in France.
The design brief for the new-generation PureTech engines called for a clean break in powertrain development. The objective was to make substantial cuts in emissions and fuel consumption, and also to reduce the weight of the engines and to make them more compact. The specifications also stipulated a specific power of 50 kW per litre.
Two different capacity versions have been developed; a 1.0 litre and a 1.2 litre with power ranging from 68hp to 82hp (50 to 60 kW). Depending on the model and the tyres fitted, combined cycle fuel consumption for the new family is between 65.7mpg and 61.4mpg (4.3 and 4.6 l/100 km) and CO2 emissions range from 99 to 107g/km.
One PureTech engine is already available in the UK – on sale now in C3 and DS3 models – combining economy plus top-level performance. The VTi 82 engine has a capacity of 1,199cc. Maximum power of 60 kW comes at 5,750 rpm for a 6 kW increase on the 1.4 litre unit it replaces. Other impressive advances have also been made with this version. Top speed is up 7mph (11km/h), and fuel economy is also improved – by up to 15.7mpg (1.5 l/100km) or 25% compared to the previous generation engine – to 62.8 mpg (4.5 l/100km). CO2 emissions are up to 35g/km lower at 104 g/km.
The VTi 68 engine, which will be available in the C3 range next year, has a capacity of 999cc. Maximum power of 50 kW at 6,000 rpm is up by 6 kW compared to the 1.1 litre engine it will replace. Progress has been made across the board, with the top speed increasing by 5mph (8km/h). Combined-cycle fuel consumption is down 17.8mpg (1.6 l/100km) to 65.7mpg (4.3 l/100km), while CO2 emissions are a full 38g/km lower at 99g/km.
The PureTech engine family features the latest technologies to meet customer expectations as closely as possible, including:
- A timing system with two overhead camshafts and four valves per cylinder, controlled directly by two mechanical push rods for enhanced fill-up and efficiency
- Continuous variable hydraulic timing for the intake and exhaust camshafts, for better performance at low and medium engine speeds and optimised fuel consumption
- On the VTi 82 version, a counter-rotating balancer shaft, for reduced vibration and quieter operation
- Electronic management to optimise battery charge, with freewheel alternator operation used to a maximum when it requires no energy expenditure.
To achieve these results, PureTech development called for a number of changes, chief among which were:
- Downsizing (reduction in capacity)
- The choice of three cylinders rather than four
- Reduced weight and more compact design
- Reduced internal friction
- Optimised combustion.
Some 52 patents were filed in the development of the new engine family, 23 concerning engine architecture, 20 engine control and 9 special manufacturing processes and tools.
Downsizing for improved efficiency
Besides reducing weight and friction, downsizing technology serves to significantly reduce fuel consumption and CO2 emissions. Already used on a number of other Brand engines, diesel and petrol included, downsizing is particularly relevant for the new engine range, which is now the entry-level offer.
To deliver at least the same performance as the previous generation, engine efficiency had to be improved to offset the reduction in the number of cylinders.
Three rather than four cylinders for multiple gains
A three-cylinder engine has a number of advantages over a four-cylinder powerplant, including reduced energy loss through friction, fewer moving parts, lower weight and more compact dimensions, all of which cut fuel consumption.
Lighter and more compact
By reducing the number of cylinders, as well as redesigning several components, the weight loss compared with the previous engines is 25kg for the VTi 68 and 21kg for the VTi 82, or over 20% for assembled engines without accessories (alternators, starters, etc.).
Using a special casting technique called investment casting, the cylinder heads, in lightweight alloy, feature a number of functions that limit weight and dimensions. The exhaust manifold is integrated directly in the cylinder head, which reduces the number of parts and overall weight. The cooling system outlet module and the upper engine support, both integrated, also figure among the parts that come directly out of casting at the Charleville foundry.
The cylinder casing, again in lightweight alloy, combined with work right from the start of the project by the design, casting and machining teams, as well as the use of digital technology in mechanics, thermomechanics and acoustics, also led to weight reduction and the integration of a number of functions. Made from pressure-cast aluminium, the cylinder casing features linings that are inserted during casting at the Mulhouse foundry.
The engine is also much more compact than the previous generation, having lost 100mm in width and 70mm in height, accessories included.
Significant reduction in friction and the number of moving parts
The decisions made during the design phase of the PureTech engine family led to developments on a number of major parts aimed at reducing friction, which accounts for roughly 20% of the power consumed by an engine.
- The main parts and components concerned were the crankshaft – and specifically its sizing – and the off-centered piston jackets
- The piston pins, segments and push rods in the timing system feature a low-friction coating (in diamond-like carbon) to keep friction down to a minimum
- The regulated, variable-capacity oil pump continually provides optimal lubrication pressure
- The timing belt, wet and housed in a casing, has the same lifespan as the car itself. This technology also helps to reduce engine noise
All these improvements make the PureTech engine the new benchmark on ultra-low friction, which has been reduced by 30% overall.
Optimised combustion for maximum reduction of fuel consumption and emissions
The combustion system, a crucial component in fuel consumption and emissions, takes full advantage of the engine technologies. The bore/stroke ratio was chosen to optimise the compromise between the intrinsic efficiency of the combustion system and engine friction loss.
The internal aerodynamics (intake ports, shape of piston heads and combustion chamber with an 11-to-1 compression ratio) were optimised to enable the combustion system to work with high rates of Internal Gas Recirculation, which reduces fuel consumption and pollutant gas emissions.
To get the engine up to temperature faster, the engine uses Split Cooling technology. When the car starts up, with the engine cold, the cooling circuits in the cylinder block and head are separated to optimise the warm-up process. This system has two advantages. One, it reduces CO2 emissions, which are higher when the engine is cold. And two, it helps to warm up the cabin faster.
For maximum efficiency, the three-way catalytic converter is located right at the engine outlet and equipped with two lambda sensors.