SUSPEND your disbelief and imagine a 463 kW Mercedes-Benz S65 AMG lining up against a 92 kW Ford Fiesta 1,0 EcoBoost at the lights in an unlikely drag race. The outcome would be easy to predict, that much is certain. But, is there a way to compare the blitzkrieg from Affalterbach with the three-time International Engine of the Year in a theoretical pound-for-pound match-up? Yes, it’s called specific engine performance. Let the fight begin.
If we take the quoted power and torque figures of each engine and divide them by engine capacity, we get the performance per capacity (a relative performance measure). We include these two figures with all our road tests and it is an indication of how stressed an engine is. Both engines are in the upper spectrums of the performance envelope, but it is clear from the table on page 110 that the EcoBoost engine just nudges ahead with 92 kW/litre and 170 N.m/litre. If we scale these numbers to the 6,0-litre capacity of the Mercedes engine, the Ford engine produces a staggering 550 kW and 1 017 N.m.
Mean piston speed
In a reciprocating engine (piston-and-crank assembly), the piston has to complete the entire stroke twice and stop at top-dead-centre (TDC) and bottom-dead-centre (BDC) positions for each revolution of the crank. At the 6 500 r/min red line of the Ford engine, this happens 108 times a second. Therefore, the average piston speed needed to cover the stroke of 82 mm twice is 17,8 m/s (or 64 km/h). The Mercedes-Benz engine revs to only 6 200 r/min but has a much longer stroke of 93 mm. The result is an average piston speed at the red line of 19,2 m/s (or 69 km/h). Therefore, the Benz engine internals are subject to higher mechanical forces owing to higher mean piston speeds (ignoring piston-mass differences). Both engines are “under-square”, which is a term that describes the bore-diameter-to-stroke length (a square engine has equal bore and stroke sizes; an example is the Toyota 86 engine, where both the bore and stroke are 86 mm). Racecar engines are “over-square”, with a much larger bore-than-stroke length to allow higher engine speeds while keeping the mean piston speed (and component stresses) low.
Brake mean effective pressure (BMEP) is a measure of the average pressure over a cycle in the combustion chamber of an internal-combustion engine to produce a brake-power figure.
The calculation is a valuable measure of an engine’s capability to do work and is independent of engine displacement; it’s therefore perfect for engine-comparison purposes.
Engine power and displacement are the main inputs used in the equation to calculate BMEP. As power is a function of torque, we calculated the BMEP at the torque peak because this is where the maximum BMEP occurs. Both engines are very similar in terms of their BMEP values, with the Ford slightly higher at 21,5 bar versus the 21,1 bar of the Benz engine. Both figures are extremely high when you take into account that a naturally aspirated petrol engine struggles to achieve a BMEP figure of more than 13 bar.
This is an excellent indicator of the potential acceleration performance and responsiveness of a vehicle. For these reasons, we include the figure in our road tests. Even though the Mercedes S65 is more than a tonne heavier than the EcoBoost-powered Fiesta, the power-to-mass figure of
213 W/kg dwarfs the 82 W/kg of the Ford. This is reflected in the claimed zero-to-
100 km/h figures of 4,3 seconds and
9,4 seconds, respectively.
The correct way to compare fuel consumption despite engine size is to calculate brake specific fuel consumption (BSFC). This calculation takes into account the power produced and fuel consumed at each speed and load point and as such denotes efficiency. BSFC maps are calculated by running a speed and load grid on an engine dynamometer, which we unfortunately could not do for this article. What we can establish, however, is how much fuel is needed to move a kilogram of
Fiesta and S65 on the same test cycle (NEDC) by dividing the fuel-consumption figure by the mass of each vehicle. The Fiesta needs only 3,8 ml/kg, while the S65 uses 5,5 ml/kg on the same cycle. This is by no means scientific, as it excludes a number of factors, but does show the Ford’s engine is more efficient on this specific test cycle.
Another quantity we could not measure is volumetric efficiency, a measure of how effectively the combustion chamber is filled with air during the intake stroke. If an engine turns at a very slow rate on a test bench (without fuelling or ignition), the volume of air that is pumped through the cylinders equals the capacity of the engine (at standard atmospheric conditions). This denotes 100% volumetric efficiency. At very high engine speeds in a naturally aspirated engine, less air is pumped through owing to air inertia and losses/restrictions in flow past the valves; volumetric efficiency can then drop to as low as 70%. In turbocharged engines where the intake pressure is above atmospheric during boost, the volumetric efficiency is above 100% because more air is pumped through than the engine volume at standard atmospheric conditions. By comparing the volumetric efficiency of two engines, you can evaluate the effectiveness of the complete intake system as well as valve-operating strategies such as overlap (where the exhaust valve stays open during the first part of the intake stroke to use some of the exhaust energy to help suck in the fresh air charge).
The winner is…
The spoils go to the Ford 1,0 EcoBoost, but the decision was based on points rather than a knockout. It’s easier for development engineers of smaller engines to make them efficient compared to larger units (see Unfair fight? below). This also supports the current trend of engine downsizing. Engines will therefore keep shrinking, but at least Mercedes has confirmed that the 6,0-litre V12 forms a part of its future model plans.
When comparing the two engines’ specific performance, the Ford appears to be the clear winner. There are several reasons why larger engines are less efficient.
• The physical mass of the engine components and resultant inertias hamper efficiency.
• Each extra cylinder adds frictional losses.
• Larger cylinder bores are more prone to knock (auto ignition) and need to run more retarded ignition timing, which is less efficient. The Benz engine uses two spark plugs per cylinder to improve the situation (24 spark plugs in total).
• Petrol engines are more thermodynamically efficient when they run closer to full load (peak torque) because there are less pumping losses compared with part-throttle running. The EcoBoost engine is much closer to full load on the NEDC test cycle (used to measure emissions and fuel consumption) than the V12.
• Larger engines have larger auxiliaries such as oil and fuel pumps, which add to the losses.