Libralato Engine for HEVs and PHEVs
In the context of ‘peak oil’, rising fuel prices and the ‘electromobility’ automotive powertrain revolution taking place, our assessment is that the Libralato engine adds most value when integrated into Hybrid Electric Vehicles (HEV) or Plug In Hybrid Electric Vehicles (PHEV)s.
An HEV or PHEV uses an electric drive train to replace or reduce dependence on the internal combustion engine (ICE), particularly at those points where the ICE operates least efficiently.
Typically efficiencies of reciprocating engines for vehicles are about 30% for gasoline (BSFC 270 g/kWh) and 40% for diesel (BSFC 200g/kWh), although state of the art engines can achieve approx 5% higher levels. A modern vehicle engine is designed to balance a complex set of conflicting parameters: torque and power, engine speeds, low emissions and fuel economy. Optimising any one of these will compromise others. The figure on the left below shows that where torque is high (acceleration), power is low and fuel consumption is high. Where power is high (speed), torque is low and fuel consumption is high. Where fuel consumption is low, torque and power are moderate. The figure on the right shows that where power is high, Carbon Monoxide (CO) emissions are high. Where torque is high Hydrocarbon (HC) emissions are high; where fuel consumption is low, Nitrogen Oxides (NOx) emissions are high. Therefore advanced catalytic converter after treatment is required.
Torque, Power, BSFC relationship and BSFC Emissions relationship
The figure below shows that a HEV can avoid use of the engine altogether in it’s least efficient operating zone. The engine can also be pushed into a more efficient operating zone whilst running at low speed, by increasing the load to generate electricity. This also has the effect of reducing CO emissions and NOx to an extent.
HEV engine efficiency
This aspect of pollutant emissions reduction is equally important as engine efficiency and CO2 reduction. UK Highways Agency 2007 statistics show very significant increases of regulated emissions and CO2 at low traffic speeds. (Graphs include an average of 10% Heavy Goods Vehicles).
Pollutant emissions v speed and CO2 emissions v speed
UK DfT 2007, statistics show that 38% of average morning peak (6:30 – 9:30am) speeds on key routes in the ten largest urban areas are 15 mph (24 km/h) or lower. Studies from across Europe show similar findings. A survey commissioned by Citroen, published in Aug 08, looked at morning rush hour traffic congestion in five major city centres: London, Cardiff, Birmingham, Norwich and Manchester. The results showed that the average driver was stopped for 25 minutes of the average hour-long commute, in which they travelled just 12.9 miles. Given this level of UK traffic delays and the data from the two figures above, it is reasonable to infer that about a quarter of the UK’s peak hours traffic is emitting over 500 g/km CO2, more than 5 times average carbon monoxide levels and nearly 3 times average NOx levels.
The figure below illustrates fuel consumption values for a typical gasoline engine.
Engine Efficiency at Key Operating Points
Using the Brake Specific Fuel Consumption figures, the engine efficiency can be calculated at key operating points:
- ~ Urban traffic 24 km/h / 15mph (bsfc: 500), engine efficiency = 16%
- ~ UK urban speed limit 50 km/h / 31mph (bsfc: 400), engine efficiency = 20%
- ~ Optimum speed 107 km/h / 66mph (bsfc: 310), engine efficiency = 26%
- ~ Highway speed 128 km/h / 80 mph (bsfc: 350), engine efficiency = 23%
Every engine is different so this graph is an approximation, but it is important to note that the engine very rarely operates at its optimum efficiency point. For the values given, this engine’s peak efficiency would be 31%.
The example below shows test data from Giant Lion Know How on a converted Ford Escape SUV (2.3L). The blue lines show the conventional vehicle’s performance over the NEDC driving cycle and the red lines show the hybrid’s performance.
Hybrid Engine Efficiency at Key Operating Points
It can be seen from the red lines that in a hybrid topology, the engine mainly operates in its peak efficiency zone irrespective of gear changes and vehicle speed (bsfc 250 – 238) = 33% - 34%.
The most polluting zones of engine operation are also avoided.
This effect can be regarded as a guiding principle in the design of HEV and PHEV powertrains, which combine the use of internal combustion engines and electric traction motors. A key design objective becomes apparent – maximise electric drive of the vehicle in slow moving urban traffic.
All light duty vehicles in Europe are tested against the New European Driving Cycle (NEDC) in order to normalise comparison. As their names suggests, the urban and extra urban parts of the cycle are designed to represent driving conditions in towns and on highways.
It is widely acknowledged that the NEDC represents ‘ideal’ conditions and the UK Government Dept DEFRA estimates that NEDC figures should generally be inflated by 15% to reflect ‘real world’ conditions4.
New European Drive Cycle
It is obvious that conventional engines need to deliver repeated short bursts of rapid acceleration in the urban cycle and more sustained high power in the extra urban cycle. This generally gives rise to higher emissions in the urban cycle. A HEV of PHEV powertrain architecture can avoid the use of the engine in slow moving (urban) traffic completely and when the engine is required, its operation can be kept within its peak efficiency zone.
When the actual motive energy requirements of the average vehicle are calculated, based on vehicle weight, speed and acceleration, frontal area, drag co-efficient, rolling resistance and angle of incline; it can be seen that average urban motoring generally requires less than 20kW of power. Highway driving generally requires less than 50kW.
LDV Average Energy Requirements
An HEV or PHEV can then use components specified to cater for these average requirements. i.e.. the ICE engine would not need to be larger than 50 kW and the electric motors could be nominally 25 kW with the ability to provide 2 – 3 times more power in short bursts < 10seconds.
The figure below is an indicative illustration of a BSFC engine map for the Libralato engine.
Indicative Illustration of Libralato BSFC Map
Since the electric motor drives the vehicle at low speed and with high torque when necessary, the engine operation could remain within its peak efficiency zone (less than 220 g/kWh, 37% - 41% efficiency), illustrated within the red circle above. This level of performance is comparable to the most advanced light duty vehicle diesel engines5 and yet the Libralato engine is predicted to be little more than 1/3rd of the size and weight of a diesel engine, at little more than 1/4 of the cost to produce. The figure below shows typical small diesel generator efficiencies for comparison.
Small diesel generator efficiencies
4Source: 2009 Guidelines to Defra / DECC’s GHG Conversion Factors for Company Reporting
5Source: As engines get much larger their efficiency increases because of improvement in the volume to surface area ratio