Method for operating an exhaust-gas-turbocharged, low-compression multi-cylinder diesel-cycle internal combustion engine

A method of operating and exhaust-gas-turbocharged, low-compression multi-cylinder Diesel-cycle internal combustion engine that has a first cylinder group which operates over the full load range, and a second cylinder group which is cut out during starting and in the idling and part load ranges but is cut in in the remaining load range, each cylinder group having associated with it a turbocharger with associated charge air pipe, the charge air pipes being interconnected by a pressure equalizing system. To improve the starting and part load behavior, and/or permit good acceleration behavior from the lower idle to rated output (to reduce emission of noxious substances in the exhaust gas), pressure equalization between the two charge air pipes is interrupted during the cylinder cutout mode - during starting as well as in the lower load and speed ranges - and is restored in the upper load range by a load-responsive device, and at the same time the intake air for the firing cylinders is preheated during the cylinder cutout mode by means known per se.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of operating an 
exhaust-gas-turbocharged, low-compression multi-cylinder Diesel-cycle 
internal combustion engine that has a first cylinder group which operates 
over the full load range, and a second cylinder group which is cut out 
during starting or at no load (idle) and in the part load range, but is 
cut in in the remaining load range. Cutting out and cutting in of the 
second cylinder group is effected by cutting off or cutting in the fuel 
supply, each cylinder group having associated with it an 
exhaust-gas-operated turbocharger with an associated charge air pipe, the 
charge air pipes communicating with each other through a so-called 
pressure equalizing system. 
A disadvantage of supercharging consists in the fact that the ignition 
pressure rises, and hence the mechanical and thermal stresses in the 
piston/connection rod/crankshaft system increase. To prevent this effect, 
supercharging must be accompanied by a corresponding decrease in the 
compression ratio of the engine. The resultant decrease in the final 
compression temperature adversely affects the ignition conditions in the 
cylinder and influences the starting and no-load behavior, including 
operation in the low-load range. Excessive formation of white smoke is the 
consequence. 
White smoke contains unburned hydrocarbons (i.e. fuel droplets) which are 
emitted with the exhaust gases from the cylinders. It is the result of 
incomplete combustion in the cylinders and occurs when the compression 
temperature in the cylinders of the engine is not sufficient to burn all 
of the injected fuel, such as is the case, for instance, when the engine 
is being started. 
White smoke thus differs in its substance from black smoke, which contains 
carbon particles and which occurs under conditions where the compression 
temperature is sufficient to ensure combustion, but the supply of oxygen 
is not sufficient to oxidize all of the carbon in the fuel. 
To improve this critical starting and part-load behavior, it is known to 
cut out a certain number of cylinders (i.e. there will be no injection 
into these cylinders). This method is generally known. It is applied to 
Otto-cycle engines to decrease the fuel consumption in the low-load range. 
The importance of this method for the supercharged Diesel engine is in the 
fact that cutting out of cylinders will place an additional load on the 
remaining firing cylinders. This means higher volumetric efficiency, 
increase in the amount of fuel injected, improved atomization or mixture 
formation, and a higher temperature level in the working cylinders. The 
result is a substantial reduction in hydrocarbon emission. 
The (straight) cylinder cutout has been applied to Diesel engines--as far 
as is known--only in no-load operation and in the transitional phase to 
low mean effective pressures. To this end, cutout control systems have 
been developed both for engines with unit and individual injection pumps 
(see, for instance, DE-PS 28 21 161, key word: Split control rod). 
Where further reduction in the compression ratios is desired, this problem 
is solved by combining cylinder cutout with post-charging of firing 
cylinders by pre-compressed air from the non-firing cylinders (see, for 
instance, German Pat. Nos. 33 22 168 and 33 23 337). 
This method calls for a considerable control complexity, including valves, 
air reservoirs, as well as cutting-out and cutting-in devices for the 
turbochargers concerned. 
The present invention therefore relies only on straight cylinder cutout, 
which itself is known. However, in the case of the type of internal 
combustion engine preferred to above with two separate turbochargers and 
two charge air pipes, which are interconnected by a pressure equalizing 
circuit, this has a serious disadvantage in poor acceleration and 
increased black smoke in the cutout mode because the charge air pressure 
built up by the relevant turbochargers via the firing cylinders expands 
instantly via the pressure equalization into the non-firing cylinder bank. 
As a result, the charge air pressure in the firing cylinders decreases 
and, consequently, is not available during combustion. As a result, a 
smoke stop provided on the injection pump (LDA=charge pressure-dependent 
full-load stop, which limits the control rod path on the injection pump 
and, consequently, the injection rate at low charge pressures) releases 
the amount of fuel needed for rapid acceleration only with such a time lag 
as is required for the necessary charge pressure to build up again or, 
failing an LDA fuel stop, would lead to substantial amounts of black 
smoke. 
The object of the present invention is therefore to offset deficient 
supercharging by the turbocharger in an internal combustion engine of the 
aforementioned general type during starting and at part load in a simple 
manner, and to achieve a sufficiently high temperature in the firing 
engine cylinders for ignition to occur despite the low compression ratio 
during starting, and/or better combustion of the fuel with reduced 
emission of noxious matter in the exhaust gas in the partial load mode.

SUMMARY OF THE INVENTION 
The method of the present invention is characterized primarily in that in 
the cylinder cutout mode--during starting as well as in the lower load and 
speed ranges--pressure equalization between the two charge air pipes is 
interrupted, and in the upper load range is restored by a load-controlled 
device, and in that at the same time, in the cylinder cutout mode, the 
intake air for the firing cylinders is preheated by means known per se, 
such as Diesel-fuel-operated glow plugs or electrically operated glow 
plugs. 
Interruption of pressure equalization in the cylinder cutout mode prevents 
charge air from being lost into the non-firing cylinders. At a suitable 
load level, the pressure equalizing circuit is opened again (in 
conjunction with the cutting in of the other cylinders) in order to supply 
both cylinder banks with the same charge pressure, because otherwise 
widely diverging cylinder peak pressures would be liable to occur due to 
the different charge pressures caused by manufacturing tolerances in the 
valve train, in gas and air passages, with regard to injection rates and 
injection timing as well as in turbine rotors and compressor impellers, 
entailing undesirably high fluctuating torques that act on the 
piston/connecting rod/crankshaft assembly and would lead to the engine 
shaking on its mountings. 
As a parallel measure, it is at the same time proposed to preheat the 
induction or intake air for the firing cylinders. This could be done under 
electronic cycling control using temperature sensors (measuring the 
temperature of the intake air) which are cut in and cut out automatically. 
Alternatively, control action as a function of the load would be 
conceivable. 
Where intercooling or aftercooling is provided, it is useful as a further 
development of the invention in solving the problem, to interrupt the flow 
of coolant through the intercooler in the cylinder cutout mode so that the 
charge air passing through the intercooler undergoes no cooling. 
A simple injection pump with only one control rod can be used for injecting 
the fuel. Cutting in and cutting off the fuel supply is effected in this 
type of pump as a function of the load by special grooves in the elements 
of the injection pump. As far as is known, this type of injection pump has 
so far been used only for interrupting the fuel supply at no load or in 
the low-load mode. As a further feature of the invention, such as 
injection pump is now proposed to be used also to interrupt the fuel 
supply during cold starting or under conditions of free acceleration (in 
the latter case in order to interrupt temporary injection of the fuel into 
the cut out cylinders). 
DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the drawing in detail, FIGS. 1 and 2 show a supercharged 
Diesel-cycle internal combustion engine that has two cylinders banks 1 and 
2, each comprising six cylinders. Supercharging of cylinder bank 1 is 
effected by the turbocharger 3, and supercharging of cylinder bank 2 is 
effected by the turbocharger 4. The charge air pipes 12 and 13 coming from 
the turbochargers are united in a diffuser-type inlet casing 5 that 
preceeds an intercooler (aftercooler) 6. Uniting the pipes in the casing 5 
would normally provide pressure equalization. According to the invention, 
the casing 5, the intercooler 6, as well as the intercooler outlet 7 are 
provided with a continuous partition 19 that has a control valve 20 in the 
region of the casing 5. The partition 19 practically creates a separate 
charge air circuit for each cylinder bank 1 and 2, with separate inlets 5a 
and 5b, separate heat exchanger units 6a and 6b, and separate outlets 7a 
and 7b, whereby controlled pressure equalization is now provided by means 
of the control valve 20 (for instance, a globe valve or rotary valve). The 
control valve 20 is controlled as a function of the load, being closed in 
the cutout cylinder mode (at least half the number of cylinders is cutout, 
i.e. one cylinder bank of the V-engine), and being open when all cylinders 
are operated. This results in good acceleration behavior from lower idle 
to rated output. On the other hand, in the upper load range the same 
charging pressure exists at all cylinder inlets, thus ensuring sufficient 
ignition pressure and equal proportions of excess air. 
As can be further seen from the figures, the charge air flows from the 
outlets 7a and 7b, via the respective inlets 14 and 15, to the cylinder 
head inlet ports, which are not shown in detail. A respective heating 
device 8 and 9 is provided downstream at the inlet of the pipes 14 and 15. 
Depending on which of the two cylinders banks is to be fired initially, 
either the heating device 8 or the heating device 9 is operated. With 
regard to heating devices, Diesel-fuel-operated glow plugs or electrically 
heated glow plugs may be used which are cut in and cut out automatically 
via a control unit under temperature or load-responsive control. In this 
manner, the intake air is preheated in a simple manner (in order to 
increase the final compression temperature). 
It should be mentioned that the exhaust gas turbines of the turbochargers 3 
and 4 are supplied in the usual manner via the exhaust gas pipes 17 and 18 
respectively, and that the fuel is delivered via the 
schematically-indicated injection pump 10, through injection pipes that 
are not shown, to the individual injectors 11 in the respective cylinder 
heads and into the cylinders which are in the firing mode (as described 
above). 
The present invention is, of course, in no way restricted to the specific 
disclosure of the specification and drawing, but also encompasses any 
modifications within the scope of the appended claims.