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Graeme Morpeth

With today’s advances in combustion technology, which engine type is right for you?

Modern Technology – Graeme Morpeth
Diesel or Gasoline?
With today’s advances in combustion technology, which engine is right for you?

Where once the scream of large-capacity, high-revving gasoline engines dominated the racetrack at Le Mans – 1999’s winning BMW LMR had a 6.0-liter V-12 – these days, the oleaginous thrum of common-rail diesel engines predominates. In the last three races, low-revving diesel engines of around 3.7 liters, integrated into hybrid power trains with electric motors driving the front wheels, powered the winning cars. So how did this revolution occur?

As is usual in these cases, there are multiple reasons: Changes in racing rules, continued development of vehicle-electronics packages, and the steady rise of diesel-engine-powered passenger vehicles across the Atlantic – “Racing improving the breed” writ large on the autobahns of Europe.
Gasoline technology
Given these trends in technologies, a comparison of diesel and gasoline engines might be useful. Both were the late 19th-century inventions of gifted engineers: Nikolaus August Otto patented the four-stroke gasoline engine in 1876 and Rudolf Diesel patented the four-stroke diesel engine in 1892. Thus was born the four-stroke’s “suck, squeeze, bang and blow” cycle.

In gasoline engines, a fuel-air mixture is ignited in the combustion chamber by means of a spark provided by … a spark plug. The resulting detonation creates heat and pressure that forces the piston down the cylinder causing the crankshaft to rotate. Initially supplying the fuel-air mixtures was a carburetor, which essentially is a venturi passage admitting a stream of air with a movable flap or throttle to control the airflow and a jet to squirt fuel into the airstream.

Not terribly complicated or efficient as a method of fueling engines, these carburetors have been supplanted first by mechanical and later by electronically controlled injection systems, often squirting fuel directly into each combustion chamber and mirroring the typical operation of diesel engines. Mercedes-Benz was the first to use mechanical direct fuel-injection systems in production automobiles with the W198 300SL Gullwing in 1954; things have evolved somewhat since then.

In modern electronic fuel-injection (EFI) system engines, much greater control of the fuel-air mix is possible to improve fuel efficiency, including:

Multiple injections per firing cycle to accommodate varying load/speed conditions
Control of the fuel-air ratios, including so-called lean burns of air-to-fuel ratios up to 22:1 or more
Cylinder deactivation modes for highway cruising in which one bank of a multicylinder engine would be subject to fuel cutoff
Stop-start technology to reduce fuel consumption in slow-moving traffic conditions and at stop lights
Integration of turbo- or super-charging devices to improve thermodynamic efficiencies and outputs

The combined results of this plethora of technological development are astonishing: When the W201 190E 2.3 was introduced in 1983, its 4-cylinder 2,299cc M102 gasoline engine produced 120 horsepower and achieved 23-31 mpg.

In 2013, with turbo-charging and stratified-charged gasoline direct-injection, the 1,796cc DE18ML used in the comparable W205 C250 produced 201 horsepower and achieved almost exactly the same fuel efficiency. The difference that highlights technical advance is that the 190E 2.3 took more than 11 seconds to reach 60 mph and topped out at 114 mph; the C250 reaches 60 mph in 7.1 seconds and has a top speed of 130 mph.
Diesel technology      
Whew! How on earth can a diesel engine compare with those numbers? Quite easily, as it happens. But why is that? To understand the difference, it is worth considering how  Rudolf Diesel’s engine works.

When first introduced in passenger cars by Mercedes-Benz in the 1936 260D, diesel engines typically compressed the air sucked into the cylinder to about 330 psi (a compression ratio of approximately 22:1) and somewhere within a few degrees of TDC (the engine’s Top Dead Center) diesel fuel was injected into the combustion chamber. At this stage, the temperature of the compressed air was above the 493 F auto-ignition temperature of diesel fuel, so when the fuel was injected, it spontaneously detonated and initiated the power – “bang” – cycle in the engine. No spark plug or electric ignition system was required.

The duration of this self-combustion process is quite long; at 5,400 rpm, which is about the upper limit for diesel engines, a 4-cylinder diesel takes 0.044 seconds to complete the power cycle. In contrast, because gasoline is much more volatile than diesel fuel and burns considerably faster, 4-cylinder gasoline motorcycle racing engines such as those manufactured by Honda can operate at over 20,000 rpm.

This relatively long diesel-ignition cycle, combined with the diesel’s greater energy density (diesel has 128,450 British Thermal Units (BTU) of energy per gallon while gasoline has 116,090 BTUs per gallon) allows the engine to produce more torque, or acceleration force, a defining characteristic of diesel engines. 

The other advantage that diesel engines had over their gasoline counterparts was simplicity; with no ignition system or pre-mix of fuel and air, they have fewer moving parts and a simple fuel-delivery system.

As a matter of interest, the gasoline carburetor and the diesel injection pump used in traditional diesel engines share one simple characteristic; their fuel output is mechanically linked to the engine speed.

In the newer common-rail direct-injection (CRD) diesel system, by divorcing engine speed and fuel supply and separating the functions of fuel-pressure generation and fuel injection, a CRD diesel system – with fuel at high pressure in a common reservoir that provides fuel to all cylinders – is able to supply fuel over a broader range of injection timings and pressures than was previously possible.

Common rail direct-injection systems were developed in the 1990s by a consortium comprised of Magneti Marelli, Centro Ricerche Fiat and Elasis, but that group lacked funds to complete the project. The design was acquired, developed and refined for mass production by the German company Robert Bosch GmbH. The system was launched in two passenger cars during 1997: the Alfa Romeo 156 2.4 JTD  (uniJet Turbo Diesel) and, more significantly, later that same year in the Mercedes-Benz W202 C220 CDI.

The evolution of the W124 and W210 models, produced in 1985-1996 and 1996-2002, provide an example of the changes made to engine outputs as common-rail technology was applied to diesel engines:

The W124 OM606.912 inline 6-cylinder engine with twin overhead cams, 24 valves and indirect mechanical injection produced 134 horsepower and 155 pound-feet of torque

As found in the W210, a turbo-charged version of this engine, the OM606.962 produced 175 horsepower and 243 pound-feet of torque

Later in its life cycle, the W210 was fitted with the OM613 twin overhead camshaft 24-valve turbo-charged V-6 common-rail diesel, thus producing 194 horsepower and 347 pound-feet of torque

In each of these models, fuel consumption improved: My own W124 E300D would return approximately 32 British mpg  or 27 U.S. mpg, while a colleague’s W210 E300TD achieved 37 British mpg or 31 U.S. mpg; and yet another’s W210 E320 CDI returned around 41/42 British mpg or 35 U.S. mpg.
Direct injection systems

So, having said all that, how do EFI and CDI systems work? The components of both systems are similar; the major difference is the pressure at which they operate. The latest-generation CRD systems work at around 2,200 bar (32,000 psi), whereas EFI systems work between 50 and 500 psi.

There are four main components in typical EFI and CDI systems – injector, high-pressure supply pump, pressure-control valve and engine control unit (ECU) – and they are roughly the same. The diesel common rail is merely a (very) thick-walled tube with screwed ports for fuel inlet and outlet and is otherwise an inert component.

The diesel fuel injectors look like conventional gasoline injectors but differ significantly; because of the very high fuel-rail pressure, they use a hydraulic servo system in normal operation. In this design, the solenoid armature does not control the injector; instead the movement of a small ball atop the injector regulates the flow of high-pressure fuel from a valve-control chamber within the injector – releasing this high pressure allows the injector to rise and deliver a charge of fuel to the engine. The injector holes are minute; their diameters can be measured in microns (1/1000mm).

Gasoline electronic fuel-injection injectors work slightly differently; the solenoid operates the injector valve directly, possible because of lower operating pressures.

Incidentally, it is the speed of operation of the diesel fuel injector – significantly increased in comparison with diesels of yesteryear – that makes the difference in a modern diesel engine’s sound. The old, distinctive clatter came from sound emitted when the cylinder pressure changed dramatically – mechanical and EFI systems fed fuel into a “pre-chamber” before the ignited fuel flowed into the main cylinder chamber – the same reason a whip cracking in the air makes a loud pop. With multiple injections per cylinder revolution, separate sounds are smoothed out between injections and a modern diesel sounds very much like a gasoline engine.

The diesel fuel pump is typically a multipiston radial design lubricated by the fuel – a radical departure from previous designs. The pump flow can be varied with engine load by closing individual pump cylinders and using a solenoid to hold the intake valve of that cylinder open. However, this causes greater fuel-delivery pressure fluctuations than when all three pistons are in operation. The volume of the common rail is such that these fluctuations in pressure, and their impact on injection cycles, are minimized. 

Gasoline EFI pumps are much simpler and less robust in their construction because they are producing much less pressure. A typical Bosch turbine pump spins at 7,000 rpm and supplies approximately 10 gpm  at 85 psi. Fuel deliveries that cause fluctuations in fuel-rail pressure are controlled by a fuel-cooled, solenoid-operated pressure-control valve.

When the pressure-control valve is not activated, its internal spring maintains a fuel pressure of about 100 bar. When the valve is activated, the force of the electromagnet aids the spring and reduces the valve opening to increase fuel pressure.

Of similar design on both gasoline and diesel engines, the fuel-pressure control valve also acts as a mechanical pressure damper, smoothing the high-frequency pressure pulses produced by the radial piston pump, especially when cylinders are deactivated. 
Gasoline or diesel, then?
With the latest in diesel direct-injection technology, there is little to choose between gasoline and diesel engines in terms of refinement, smoothness or interior noise. The difference is seen at the pumps; in general, modern diesel cars return between 10 and 30 percent or better fuel economy than their gasoline counterparts. In the current E-Class as sold in the United States, Mercedes-Benz Cars offers an inline 4-cylinder diesel engine and a V-6 gasoline engine. Acceleration response or economy? The choice is yours.
Here are the comparable statistics:
2014 E-Class Technical Data
E250 BlueTEC 4Matic  Inline 4 Turbodiesel  
Power: 195 hp   Torque: 369 lb-ft    0-60: 8.2 sec estimated   Fuel economy: 27/38 mpg      
E350 Sedan 4Matic V-6 non-Turbo Direct Injection Gasoline         
Power: 302 hp   Torque: 273 lb-ft    0-60: 6.6 sec estimated   Fuel economy: 20/28 mpg