Modern
Technology
Graeme
Morpeth
TECHNIC AL
Diesel or Gasoline?
With today’s advances in
combustion technology,
which engine is right for you?
W
here 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
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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 slowmoving traffic conditions and at stop lights
Integration of turbo- or super-charging devices to improve
thermodynamic efficiencies and outputs
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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.
Diesel power has come a long way since it was invented by Rudolf Diesel in the
1890s. Mercedes-Benz first began to employ diesel engines in cars in 1936;
contemporary powerplants such as the S400 CDI are both refined and powerful.
TECHNIC AL
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TECHNIC AL
Who says you can’t have it all? A modern diesel like this 2014 Mercedes-Benz E250 BlueTEC offers a compelling mix of power, performance and economy.
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
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As found in the W210, a turbo-charged version of this engine,
the OM606.962 produced 175 horsepower and 243 poundfeet 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
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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 pressurecontrol 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 fuelpressure 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. Here are the comparable statistics:
Gasoline or Diesel? 2014 E-Class Technical Data
Model
Engine
Power Torque 0-60
Economy
E250 BlueTEC Inline 4
4Matic
Turbodiesel
195 hp 369 lb-ft 8.2 sec
estimated
27/38
E350 Sedan V-6 non-Turbo 302 hp 273 lb-ft 6.6 sec
4Matic
Direct Injectionestimated
20/28
Acceleration response or economy? The choice is yours.