U.S. patent application number 10/444929 was filed with the patent office on 2004-11-25 for diesel engine with intake cam phaser for compression ratio control.
Invention is credited to El Tahry, Sherif Hussein, Krieger, Roger B..
Application Number | 20040231623 10/444929 |
Document ID | / |
Family ID | 33450780 |
Filed Date | 2004-11-25 |
United States Patent
Application |
20040231623 |
Kind Code |
A1 |
El Tahry, Sherif Hussein ;
et al. |
November 25, 2004 |
DIESEL ENGINE WITH INTAKE CAM PHASER FOR COMPRESSION RATIO
CONTROL
Abstract
A cam phaser is mounted on the intake camshaft of a diesel
engine and is operable to selectively retard timing of only the
intake valves relative to the crankshaft. The purpose of retarding
timing of the intake valves is to retard valve closing sufficiently
to shorten the effective compression strokes of the pistons and
thus reduce the effective compression ratio. This occurs when the
intake valves remain open past piston bottom dead center for a
desired period into the normal compression stroke phase of engine
operation. This reduces compression pressures so that combustion
temperatures are reduced and exhaust emissions, primarily NOx, may
be thus limited under conditions of warmed-up engine operation.
Inventors: |
El Tahry, Sherif Hussein;
(Troy, MI) ; Krieger, Roger B.; (Birmingham,
MI) |
Correspondence
Address: |
KATHRYN A MARRA
General Motors Corporation
Legal Staff, Mail Code 482-C23-B21
P.O. Box 300
Detroit
MI
48265-3000
US
|
Family ID: |
33450780 |
Appl. No.: |
10/444929 |
Filed: |
May 23, 2003 |
Current U.S.
Class: |
123/90.15 |
Current CPC
Class: |
F02D 13/0238 20130101;
F01L 1/022 20130101; Y02T 10/12 20130101; F02D 2041/001 20130101;
F02D 41/06 20130101; Y02T 10/142 20130101; F01L 2001/0537 20130101;
F02D 13/0269 20130101 |
Class at
Publication: |
123/090.15 |
International
Class: |
F01L 001/34 |
Claims
1. A diesel engine having cylinders and pistons defining expansible
combustion chambers into which combustion supporting gas is
compressed during compression strokes of the pistons for
compression ignition and burning of injected fuel to drive a
crankshaft, intake and exhaust valves actuated by crankshaft driven
intake and exhaust camshafts for controlling the timed admission of
air to and the discharge of exhaust from the combustion chambers,
and the improvement comprising: a cam phaser on only the intake
camshaft and operable to selectively retard timing of only the
intake valves relative to the crankshaft to retard intake valve
closing sufficiently to shorten the effective compression stroke,
thereby reducing the effective compression ratio and lowering
combustion temperatures in the combustion chambers for controlling
exhaust emissions under predetermined conditions of operation; and
a control effective to vary intake valve timing from a nominal
setting to a retarded closing timing of up to 70 degrees of
crankshaft rotation.
2. A diesel engine as in claim 1 wherein the control is effective
to maintain a nominal high compression ratio for cold engine
starting and warm-up and to reduce the effective compression ratio
for operation at warmed-up engine conditions to limit combustion
temperatures during normal engine operation.
3. A diesel engine as in claim 2 wherein the control is responsive
to combustion chamber temperatures.
4. A diesel engine as in claim 2 wherein the control is responsive
to combustion chamber pressures.
5. (canceled)
6. A method of operating a supercharged diesel engine for
controlling exhaust emissions during selected conditions of engine
operation, the method including the steps of: providing a cam
phaser connected with an intake camshaft and operable to
selectively retard intake cam timing for controlling timing of the
intake valves; operating the engine with a nominal cam timing for
cold engine starting and warm-up; and actuating the cam phaser to
retard intake cam timing under warmed-up engine conditions to
provide preselected reduction of effective piston compression ratio
of cylinder charge gases in order to lower NOx exhaust emissions;
and boosting the pressure of the cylinder charge gases to provide a
trapped charge mass equivalent to the charge mass without reduction
of the compression ratio; whereby NOx emissions are further reduced
due to reduced combustion temperatures.
7. (canceled)
8. (canceled)
Description
TECHNICAL FIELD
[0001] This invention relates to diesel engines and, more
particularly, to control of cylinder compression ratio using an
intake cam phaser.
BACKGROUND OF THE INVENTION
[0002] It is known in the art to provide means for varying the
compression ratio of a diesel engine in order to provide a
relatively high compression ratio for cold starting and warm-up,
where compression ignition is more difficult, and to provide
reduced compression ratios for operating in other modes,
particularly at high loads and speeds, to reduce peak combustion
pressures and temperatures. Recently the emphasis for such
arrangements is primarily to minimize emissions of nitrogen oxides
(NOx) by operating at lower compression ratios where this is
possible. Many devices have been proposed for compression ratio
variation, including variable valve timing mechanisms and engine
components such as pistons and cylinder heads with movable
combustion chamber walls. In general these devices are relatively
complex and add significant cost to the manufacture of an
engine.
[0003] In spark ignition engines, cam phasers are known as simple
devices for varying cam timing and thus changing valve timing to
the extent permitted by the camshaft layout. These devices normally
provide for advancing or retarding the cam timing in order to
obtain desirable combustion characteristics. It is believed that
cam phasers have not been utilized on diesel engines because the
piston to cylinder head clearance is so small that altering intake
and exhaust valve timing may result in contact of the pistons with
the valves. A simple and relatively low cost apparatus and method
for controlling compression ratio in a diesel engine is
desired.
SUMMARY OF THE INVENTION
[0004] The present invention provides a desired engine combination
by the addition of a cam phaser capable of retarding the timing of
only the intake valves of a diesel engine in order to reduce its
compression ratio. A typical diesel engine has cylinders and
pistons defining expansible combustion chambers into which air is
admitted and compressed during compression strokes of the pistons.
Compression increases the air temperature so that injected fuel is
self-ignited and burns, creating power to drive a crankshaft.
Intake and exhaust valves, actuated by separate crankshaft driven
intake and exhaust camshafts, control timed admission of air to and
discharge of exhaust products from the combustion chambers.
[0005] In accordance with the invention, a cam phaser is mounted on
the intake camshaft and is operable to selectively retard timing of
only the intake valves relative to the crankshaft. The purpose of
retarding timing of the intake valves is to retard valve closing
sufficiently to shorten the effective compression strokes of the
pistons and thus reduce the effective compression ratio. This
occurs when the intake valves remain open past piston bottom dead
center for a desired period into the normal compression stroke
phase of engine operation. This reduces compression pressures in
the combustion chambers so that combustion temperatures are reduced
and exhaust emissions, primarily NOx, may be thus limited under
conditions of warmed-up engine operation.
[0006] Additional reductions in combustion temperatures can be
achieved, in conjunction with use of an intake cam phaser in
turbocharged or supercharged diesel engines, by increasing the
intake boost pressure to maintain constant trapped air mass in the
cylinder, even when intake camshaft phase delay is utilized. This
approach allows maintaining lower combustion temperatures, thus
inhibiting NOx formation by preventing increases in fuel-air ratio
as compression ratio is decreased.
[0007] These and other features and advantages of the invention
will be more fully understood from the following description of
certain specific embodiments of the invention taken together with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a pictorial view of a diesel engine having
portions broken away to show the interior construction including
application of a cam phaser on the intake camshaft;
[0009] FIG. 2 is an enlarged pictorial view illustrating a portion
of the intake camshaft together with the intake valves and cam
phaser; and
[0010] FIG. 3 is a valve lift diagram showing the variation in
intake cam timing by the cam phaser.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0011] Referring now to the drawings in detail, numeral 10
generally indicates a diesel engine having a variable compression
ratio in accordance with the invention. Engine 10 conventionally
includes a plurality of cylinders 12 having therein reciprocable
pistons 14 connected with a crankshaft 16. The ends of the cylinder
are closed by a cylinder head 18 so that the cylinders and pistons
define expansible combustion chambers 20.
[0012] The cylinder head is provided with intake valves 22 which
control the timing of intake air into the cylinders during intake
strokes of the pistons. Exhaust valves 24 in the cylinder head
control timing of the discharge of exhaust products from the
combustion chambers during exhaust strokes of the pistons. In the
engine shown there are two intake valves and two exhaust valves for
each cylinder, however, any suitable number of valves provided for
operation of the engine may be utilized in accordance with the
invention.
[0013] The intake and the exhaust valves are actuated by separate
intake and exhaust camshafts 26, 28. The intake and exhaust
camshafts exclusively operate their respective intake and exhaust
valves, however, both are driven by the crankshaft 16 through a
timing chain 30.
[0014] Referring to FIG. 2, there is shown an enlarged view of a
portion of the intake camshaft 26 including cams 32 which engage
hydraulic valve lifters 34 to actuate the intake valves 22. A cam
phaser 36 is mounted on an outer end of the intake camshaft 26 and
is connected with a control 38. Through internal passages 40,
indicated schematically, the control 38 provides pressurized oil to
the cam phaser as needed to alter timing of the intake valves by
retarding or advancing their angular position relative to the phase
angle of the camshaft.
[0015] Referring now to FIG.3 of the drawings, there is illustrated
a valve timing diagram. The lift motions of the valves are
illustrated by an exhaust curve 42 and by a pair of intake curves
44, 46. As illustrated, exhaust valve opening begins at about 100
degrees after top dead center (ATDC) and rises on lift curve 48 to
a peak at about 230 degrees ATDC. Thereafter, exhaust valve closing
proceeds down closing curve 50 to complete closing at about 370
degrees ATDC.
[0016] The nominal intake curve 44 begins intake valve opening at
about 330 degrees ATDC and proceeds along lift curve 52 to a peak
at about 460 degrees ATDC. Thereafter, the nominal intake curve
proceeds down closing curve 54 to valve closing at slightly after
600 degrees ATDC. Operation with this nominal valve timing provides
a relatively high compression ratio in the engine which may
approximate 15/1 to 20/1 depending on the design of the particular
engine.
[0017] In operation, the nominal mode of operation is utilized for
cold engine starting and warm-up. This is necessary because the
intake air charge must be compressed to a gas temperature high
enough to provide reliable and consistent compression ignition of
fuel injected into the combustion chambers near their piston top
dead center positions. After the engine is warmed up and the
cylinder and piston walls are heated, reduction of the compression
ratio to a lower range, such as 12/1 to 15/1 depending on the
engine configuration, can be utilized to provide effective
compression ignition to operate with reduced combustion
temperatures in order to control or limit NOx emissions.
[0018] Such reduction of combustion temperatures and emissions is
obtained by reducing the engine compression ratio by the method of
actuating the cam phaser 36 to retard the intake valve timing, as
shown by the intake curve 46. With this cam timing, intake valve
lift starts slightly before 400 degrees ATDC, about 50 degrees
after closing of the exhaust valve. Valve lift proceeds along a
lift curve 56 to a peak lift at about 550 degrees ATDC and then
along a closing curve 58 to intake valve closing at about 650
degrees ATDC.
[0019] With this retarded timing, the intake valve closing is
delayed until about 70 degrees before top dead center (TDC) of the
respective pistons. Thus, the effective compression stroke is
shortened by about 50 degrees from that of the nominal intake valve
lift curve 44. The result is that the effective compression ratio
of the engine is reduced.
[0020] While this will provide reduced combustion temperatures
resulting in a reduction of NOx emissions, the effect is limited by
fuel heating of the smaller gas charge. With a turbocharged or
supercharged engine, the boost level may be increased to provide a
trapped mass of the intake gas charge, including air and exhaust
gases if needed, that is equivalent to the mass provided without
the reduced compression ratio. Burning and expansion of the larger
charge with the reduced compression ratio then results in a greater
temperature reduction and a resulting greater reduction in NOx
emissions.
[0021] When the engine is again operated at light loads or during
starting and warm-up, the cam phaser is again returned to the
initial nominal position and compression ratio is again increased
so that dependable compression ignition of the intake air fuel
charge is obtained.
[0022] In order to use a cam phaser in the manner outlined for
reducing the effective compression ratio and resulting compression
temperatures of a diesel engine, the engine intake and exhaust
valves must be operated by separate camshafts so that only the
intake valves are retarded. The variations in valve timing for
which cam phasers are utilized in spark ignition engines are not
generally usable in diesel engines because the exhaust valve timing
cannot be retarded nor the intake valve timing be advanced without
the pistons contacting the valves due to the low piston to head
clearance.
[0023] Thus, the application of cam phasers to a diesel engine is
not known to have previously been considered practical. However,
the use in the present invention, where only retarding of the
intake valves from their nominal timing is utilized, provides a
simple and low cost method of controlling combustion temperatures
and controlling NOx emissions in warmed-up operation of a diesel
engine.
[0024] While the invention has been described by reference to
certain preferred embodiments, it should be understood that
numerous changes could be made within the spirit and scope of the
inventive concepts described. Accordingly, it is intended that the
invention not be limited to the disclosed embodiments, but that it
have the full scope permitted by the language of the following
claims.
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