Title:
Continually variable valve timing, lift, and duration for internal combustion engine
Kind Code:
A1


Abstract:
Continually variable valve timing, lift and duration in an engine are achieved by altering the location of the pivot of the rocker arm and by using a mechanical lash compensator. The pivot of the rocker arm alters when a variable valve lift disc rotates. The lash compensator comprises a cylindrical plunger inserted in a body. The plunger and the body are separated by a spring. The separation gap or lash varies depending on the amount of rocker arm pivot movement. The larger amount of lash results in lower valve lift and small amount of lash results in higher valve lift. The variable lift disc is used to alter the position of the rocker arm pivot and it has engaging teeth that mate with a helical gear. The rocker arm pivot shaft is mounted on the variable lift disc, when the disc rotates, it alters the rocker arm pivot position. The disc rotates under the longitudinal movement of the helical gear controlled by the hydraulic oil pressure from lubricant. The longitudinal travel of the helical gear is directly related to the engine speed and the hydraulic pressure. Under higher engine RPM, higher hydraulic pressure pushes helical gear longitudinally and rotates the variable lift disc, thus achieve higher valve lift by eliminating the lash between the plunger and the body. Under low engine RPM or low hydraulic oil pressure, a spring attached to the helical gear assembly forces the helical gear subsequently the pivot of rocker arm back to its original position. Therefore, the lash in the lash compensator increases, consequently the valve lift decreases. The valve timing, lift, and duration vary continuously depending on engine speed.



Inventors:
Wang, Yushu (Marshall, MI, US)
Application Number:
10/042021
Publication Date:
07/10/2003
Filing Date:
01/10/2002
Assignee:
WANG YUSHU
Primary Class:
Other Classes:
123/90.27, 123/90.44
International Classes:
F01L1/18; F01L1/34; F01L13/00; (IPC1-7): F01L1/34; F01L1/02; F01L1/18
View Patent Images:
Related US Applications:



Primary Examiner:
CORRIGAN, JAIME W
Attorney, Agent or Firm:
Yushu Wang (6 Friendship Lane, Marshall, MI, 49068, US)
Claims:

I claim:



1. A continually variable valve timing, lift and duration system for an internal combustion engine comprises a lash compensator, a rocker arm, a variable lift disc, and a driving device.

2. The system as claimed in claim 1 wherein the said lash compensator comprises a cylindrical plunger, a body and a spring. The said plunger is inserted into the said body. The said plunger and the said body are separated by the said spring. The separation gap or lash varies depending on the rocker arm pivot position. Larger lash at cam base circle results in low valve lift, and smaller lash results in greater valve lift.

3. The system as claimed in claim 1 wherein the said rocker arm is end pivoted and the pivot is movable.

4. The system as claimed in claim 1 wherein the said rocker arm is center pivoted and the pivot is movable.

5. The system as claimed in claim 1 wherein the pivot of rocker arm is mounted to the said variable lift discs. The said disc is toothed on inside diameter. The position of the said rocker arm pivot alters when the said disc rotates.

6. The system as claimed in claim 1 wherein the said pivot point of rocker arm is mounted to the said variable lift discs. The said disc has engaging gear teeth on outside diameter. The position of the said rocker arm pivot alters when the said disc rotates.

7. The system as claimed in claim 1 wherein the said variable lift disc can be a section of a circular disc.

8. The system as claimed in claim 1 wherein the said variable lift disc can be a lever.

9. The system as claimed in claim 1 wherein the said driving device comprises a gear, a spring, and a hydraulic system. The said gear makes the said variable lift disc rotate when the said gear moved by the said hydraulic system. The said spring pushes the said helical gear back towards its original position when the pressure of the said hydraulic system drops. And the pressure of said hydraulic system is a function of engine speed.

10. The system as claimed in claim 1 wherein the said driving device is a stepping motor. The motor drives the variable disc and alters the rocker arm pivot position, thus alters the valve lift.

11. The system as claimed in claim 7 wherein the said spring is replaced with a hydraulic unit. The pressure difference of said hydraulic system drives the gear and rotates the variable disc. The pressure difference of said hydraulic system is a function of engine speed.

Description:

FIELD OF THE INVENTION

[0001] The present invention relates to an internal combustion engine using poppet type valves to direct gases into and out of one or more cylinders. More specifically, the present invention addresses the variable valve timing, lift and duration mechanism of internal combustion engines. The degree of valve lift and duration may be continuously altered depending on the engine speed, along with the opening and closing times of the valves, so that engine torque at different engine speeds, fuel efficiency, emission, idle stability, and valvetrain wear characteristics are all improved.

BACKGROUND OF THE INVENTION

[0002] The flow dynamics of air or air/gas mixture entering and exiting internal combustion engines is one of the controlling factors of engine performance. Most engines must work over a wide speed and load range, making it difficult to achieve optimum efficiency over more than a narrow part of that range. For simplicity, economy and durability, most conventional four stroke engines use the tried-and-true fixed camshaft systems that have constant phase (when the valves are opened), duration (how long the valves are held open) and lift (how far the valves are lifted off their seats). This leads to certain design compromises to achieve acceptable performance. An engine that produces high torque for its capacity at low engine speeds usually gives poor torque at higher engine speeds, and vice versa. In a paper published by the Society of Automotive Engineers (Hara, Kumagai and Matsumoto, 1989, SAE paper 890681), the authors present experimental results on an engine in which the timing and lift were varied. Torque was improved by 7% at 1600 rpm by variation of lift, and the improvement at 6000 rpm was 14%. Alteration of lift of the intake valve produced most of the effects seen.

[0003] Many approaches have been proposed and tried in attempts to optimize the flow processes. Improvements to the flow dynamics are achieved by three separate but interrelated processes: variable phase, variable duration, and variable lift. It is well known that engines that produce high torque at low speeds have lower overlap between the closing of the exhaust valve and opening of the intake valve. Small overlap allows for little communication between the exhaust gases and the incoming fresh charge, limiting the amount of uncontrolled mixing. This leads to stable operation. However, at high speeds the inertia of the gases requires a greater period of overlap to allow for gas exchange. The simplest way of achieving the change in overlap is to alter the relative timing, or phasing, of the intake camshaft to the crankshaft and exhaust camshaft.

[0004] If the phase of a valve event is altered, say advancing the valve opening to an earlier crankshaft angle, then the closure of that valve is also advanced. In many cases this causes a reduction of the amount of combustible gas that can enter the engine. To overcome this situation, the duration of the valve event may be altered. In the example above, as the engine speed is increased and the valve overlap is increased (opening the intake valve earlier), the period that the intake valve stays open is extended to delay the closing.

[0005] The peak lift of valves is designed to accommodate gas flow at maximum engine speeds without significant pressure drops. This is more important for the intake process than the exhaust process, where the piston pushes the gases out. At engine speeds below maximum, the velocity of incoming gases through the valve curtain will produce less turbulence, and may lead to lower torque than would be achieved with a smaller valve opening. By varying valve lift with engine speed, torque may be enhanced over the entire operating range of the engine. Additionally, reduced valve lifts at lower speeds reduce the frictional losses of the valve train and reduce valvetrain wear as well, depending on the design.

[0006] There are many examples in the U.S. patent literature of methods of varying either or all of phase, duration and lift. Many authors have recognized that engine performance over a wide speed range may be improved by providing a means of switching between two independent cam profiles for low and high speed operation. Such an “on or off” type controller will provide different values of phase, duration and lift between the two (or possibly more) different engine speed ranges, resulting in improved performance and efficiency for each speed range. However, within each speed range, there is no means or varying phase, duration and/or lift. Examples of such mechanisms are given in U.S. Pat. Nos. 2,934,053 by Longenecker, 4,151,817 by Mueller, 4,205,634 by Tourtelot, 4,970,997 by Inoue, et al., and 5,113,813 by Rosa.

[0007] Variable valve timing, lift and duration have found many commercial applications in the last decade. Honda's latest 3-stage VTEC (Variable Valve Timing and Lift Electronic Control) system has three cam lobes with different timing and lift profile, one has fast timing and high lift, the second has slow timing and medium lift, and the third has slow timing and low lift. During low rpm operations, the rocker arms riding the low rpm lobes push directly on the top of the valves. In most of the cam profiles of the two intakes valves will be slightly different, promoting swirl in the combustion chamber for better driveability. At high rpm, the ECU sends a signal to an oil control valve that allows pressure to flow into the low rpm rocker arm. A third, high rpm rocker arm sits between two low rpm arms and follows a much more aggressive lobe. When oil pressure arrives, hardened steel pins pop out of the sides of the low rpm rocker arms and slide into sockets in the high rpm arm, and the valves start following the larger cam profile. Nissan Neo VVL has very similar 3-stage lift mechanism. Toyota's VVTi system uses slide pin to latch and unlatch the hydraulic lift to vary the valve lift for the direct acting type valvetrain. For rocker arm type valvetrain, it uses a sliding pad to latch and unlatch, so achieve the 2-stage valve lift. There are many more manufacturers using the similar discrete lift mechanism for variable valve lift and duration and using separate cam phasing for variable valve timing. BMW however has a mechanical system called Valvetronic that uses conventional lobes. It also uses a secondary eccentric shaft with a series of levers and roller followers, activated by a stepper motor. The stepper motor changes the phase of the eccentric cam, modifying the rocker ratio of the rocker system to achieve a continuous variable valve lift and duration. There are many components added to the Valvetronic valvetrain system and the stiffness of valvetrain is significantly reduced. This is expected to pose problem for valvetrain dynamics at high speeds.

[0008] Another means of achieving variation in all three parameters is to use an axially moveable camshaft, with a variable profile in the axial direction. In this case there may be a smooth transition between different values of phase, duration and lift, although the relationship between all three is again fixed for a particular axial position of the camshaft. U.S. Pat. Nos. 3,618,574 by Miller and 5,080,055 by Komatsu, et al., describe such devices.

[0009] An alternative approach to varying all three parameters involves the use of multi-part rocker arms, with one or more of the arms pivoted eccentrically. In U.S. Pat. No. 4,297,270 by Aoyama two interacting rocker arms function to vary phase, duration and lift.

[0010] In U. S. Pat. No. 4,714,057 by Wichart, the author discloses control over all three parameters by using a multi-part rocker arm, and a control cam as well as the lift cam. A major purpose of their invention is to be able to control engine load without a throttle plate.

[0011] An innovative scheme is disclosed in U.S. Pat. No. 4,898,130 by Parsons, to vary the phase, duration and lift of the valves, with an eccentrically mounted oscillating drive.

[0012] Variable valve lift is achieved by yet another means in U.S. Pat. No. 5,031,584 by Frost. Two fixed pivot rocker arms are combined with a movable interposed member to alter the mechanical advantage of the camshaft to valve movement. Another means achieving variable valve lift by moving the pivot point is given by Hoffman in U.S. Pat. No. 5,205,247. A rotatable pivot shaft locates a pivot point for a circular rocker arm. The centers of the circular arms of the rocker arm are located on the same side of the rocker arm as the pivot. As the pivot point is varied, the circular shape of the rocker arm offers the same geometry to the cam and valve at each location of the pivot. The valve timing is altered by using different radii and/or offset centers for the arc segments either side of the pivot, combined with cam profiles that differ from standard profiles.

[0013] Entzminger offers a simple concept for varying valve lift in U.S. Pat. No. 4,721,007. A toothed pivot shaft mates with a toothed rack embedded in an elongate rectangular slot in the rocker arm. The pivot shaft translates and rotates simultaneously, following a linear path defined by another stationary toothed rack.

[0014] Another class of actuation mechanisms that can vary lift and duration is that of hydraulic actuation, with lost motion. In this method, the cam follower allows enclosed hydraulic fluid to leak out either through a fixed orifice, or through a controlled orifice. For the passive mechanism, the result is that the valve will not open as far or as long at low engine speeds, while at high speeds the leakage is insufficient to significantly alter the valve movement from a conventional system. The active control approach allows lift and duration to be controlled more closely. The result is that conventional throttling may be discarded, as valve motion may be enough alone to control the intake charge. Such a system is described in SAE Paper 930820 (Urata, et al., 1993).

[0015] It has been recognized that non-variable valve duration is no more acceptable, from the point of view of engine efficiency. Variable valve duration and lift, even limited at only two or three stages of the speed spectrum of an engine, demonstrates, among other advantages, the superior capability of torque and emissions control. Obviously, valve duration and lift that is optimal at every point on the engine speed scale, and for all conditions of engine operation, would be proportionately superior to a two or three stage system as discussed. Of the many systems proposed to achieve variable valve duration, the proposed system offers a predictable baseline of induction and exhaust control throughout the engine speed range, it offers opportunities to maximize fuel usage, and minimize polluting emissions, factors of crucial importance today and into the future.

SUMMARY OF THE INVENTION

[0016] An object of the present invention is to provide a continuously variable valve timing, lift and duration in an internal combustion engine.

[0017] To achieve this object, it is proposed that the system shall comprise a lash compensator, a rocker arm with movable pivot, a variable disc, and a driving device.

[0018] The said lash compensator consists of a body, a plunger inserted into the body, and a spring separating the body and the plunger. The separation gap or lash between the plunger and the body varies and compensates the amount of valve lift when the rocker arm pivot moves, therefore alters the valve timing, lift and duration accordingly.

[0019] The movable rocker arm pivot is connected to the variable lift disc which has teeth either on inside or outside diameter. The rocker arm pivot alters when the disc rotates through mating gear of a driving device. The amount of rocker arm pivot movement depends on the engine speed. The driving device can be either a hydraulic pressure system or a stepping motor system. The combination of the rocker arm pivot position change and the mechanical lash between the plunger and the body in the lash compensator controls the degree of valve timing, lift and duration.

[0020] This continuously variable valve timing, lift and duration system is expected to achieve the improved fuel economy, emission, engine performance, and valvetrain wear characteristics throughout the engine speed range.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a schematic of continuously variable valve timing, lift, and duration device with an end pivoted rocker arm

[0022] FIG. 2 is a schematic of continuously variable valve timing, lift, and duration system with an end pivoted rocker arm illustrating low valve lift. Note the separation gap h between the plunger and the body in the lash compensator is large and separated by a spring

[0023] FIG. 3 is a schematic of continuously variable valve timing, lift, and duration system with an end pivoted rocker arm illustrating peak valve lift. Note the gap h between the plunger and the body in the lash compensator separated by a spring is diminished due to the rocker arm pivot movement

[0024] FIG. 4 is a schematic of variable disc with engaging gear teeth on inside diameter. The disc rotates when the helical gear moves longitudinally through the inside diameter gear teeth. The helical gear is driven by a hydraulic system. The spring returns the gear and disc to its original position when the hydraulic pressure drops.

[0025] FIG. 5 is a schematic of continuously variable valve timing, lift, and duration device with a center pivoted rocker arm

[0026] FIG. 6 is a schematic of continuously variable valve timing, lift, and duration system with a center pivoted rocker arm illustrating low valve lift. Note the separation gap h between the plunger and the body in the lash compensator is large and separated by a spring

[0027] FIG. 7 is a schematic of continuously variable valve timing, lift, and duration system with center pivoted rocker arm illustrating peak valve lift. Note the gap h between the plunger and the body in the lash compensator separated by a spring is diminished due to the rocker arm pivot movement

[0028] FIG. 8 is a schematic of variable disc toothed on outside diameter and driven by a helical gear

[0029] FIG. 9 is a schematic of another variable disc example with gear teeth on outside diameter

[0030] FIG. 10 is a schematic of yet another variable disc example with gear teeth on outside diameter

[0031] FIG. 11 is a normal valve lift diagram, neglecting the ramping at valve opening and closing

[0032] FIG. 12 is a conventional phasing diagram when the intake is advanced

[0033] FIG. 13 is a conventional phasing diagram when the intake is advanced and the exhaust is retarded

[0034] FIG. 14 is the lift diagram of continuously variable valve timing, lift and duration system when cam phasing is set at peak valve lift, i.e. no lash in the lash compensator when intake valve opens at top dead center (TDC) and valve has peak valve lift. Note that timing is retarded and duration reduced as valve lift decreases

[0035] FIG. 15 is the lift diagram of continuously variable valve timing, lift and duration system when cam phasing is set at minimum valve lift, i.e., the lash in the lash compensator is maximum when intake valve opens at top dead center (TDC) and valve has minimum lift. Note that there is no overlap at minimum lift, but the overlap and duration increase as the valve lift increases.

[0036] FIG. 16 is the continuously variable valve timing, lift and duration phasing diagram when the intake advanced and the exhaust retarded at minimum valve lift setting. The cam timing is set for both exhaust valve closes and intake valve opens at top dead center (TDC) when the lash in the lash compensator is maximum at cam base circle. Note also that there is no overlap at minimum lift, but the overlap and duration increase as the lift increases.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] FIG. 1 shows a preferred embodiment of the present invention depicting the end pivoted roller rocker 3 with movable pivot 4, the lash compensator 13 with adjustable lash, and the variable disc 7 with teeth on inside diameter. In practice, camshaft 2 rotates in synchronism with the engine crankshaft so as to displace cam follower 3 which actuates the valve 1. The said rocker arm 3 has the pivot shaft 4 at one end and the other end actuates the valve 1 through the said lash compensator 13. The said lash compensator 13 consists of a cylindrical plunger 5 inserted in a body 6. A spring 12 separates the said plunger 5 and body 6, thus provides adjustable lash, FIG. 2 and FIG. 3. When the pivot 4 of the rocker arm 3 moves downwards, spring 12 push the plunger 5 and the body 6 apart, FIG. 12. The rate of spring is not high enough to open the valve. When camshaft 2 rotates, cam lobe pushes the plunger 5 down to close the gap h between the plunger 5 and the body 6 before it can lift the valve 1. Therefore, when the rocker arm shaft moves down, the valve lift is getting smaller, duration getting shorter and timing retarded i.e., opens late but close earlier. When the pivot 4 of the rocker arm 3 moves up, the gap h between plunger 5 and the body 6 is closing up. The gap h fully closes when the rocker shaft 4 moves up to a preset limit, the valve lift reaches its peak lift under a specific cam profile, FIG. 3. The pivot 4 of the rocker arm 3 can be altered using a device rotatable within predetermined limits by any suitable control system including hydraulic, electrical and mechanical means and the like. FIG. 4 shows details of an example of such mechanism with a variable lift disc 7 and helical gear 8, both are used together to alter the rocker arm pivot 4 position. Under hydraulic pressure 15, the helical gear moves longitudinally and rotates the variable lift disc. It therefore alters the pivot of rocker arm assembly. The spring 10 and/or 11 is used to push back the helical gear back, thus changes the rocker arm pivot position back to low valve lift. The spring 10 and 11 can be replaced with a hydraulic chamber. The amount of disc rotation, rocker arm pivot movement, helical gear travelling is a function of hydraulic pressure or engine speed.

[0038] FIG. 5 shows a second preferred embodiment of the present invention depicting a center pivoted rocker arm with a lash compensator, a rocker arm with movable pivot, and a variable disc. In practice, camshaft 2 rotates in synchronism with the engine crankshaft so as to displace cam follower 3 which actuates the valve 1. The rocker arm pivot shaft 4 is located in the middle. One end of the arm contacts the cam lobe 2 and the other end actuates the valve 1 through the lash compensator 13. The lash compensator 13 can be positioned at the other end of the rocker arm 3 as well. The lash compensator 13 consists of a cylindrical plunger 5 inserted into a body 6 and separated by a spring 12, FIG. 6 and FIG. 7. When the pivot 4 of the rocker arm 3 moves upwards, spring 12 pushes the plunger 5 and the body 6 apart. When camshaft 2 rotates, cam lobe pushes the lash compensator to close the gap h between the plunger 5 and the body 6 before it can lift the valve 1. Therefore, when the rocker arm pivot moves up, the valve lift is getting smaller, duration getting shorter and timing retarded i.e., opens late but close earlier. When the pivot 4 of rocker arm 3 moves downwards, the gap h between the plunger 5 and the body 6 is closing up. The gap h fully closes when the rocker pivot moves up to a preset limit, the valve lift reaches its peak lift under a specific cam profile, FIG. 7. The pivot 4 of the rocker arm 3 can be altered using a device rotatable within predetermined limits by any suitable control system including hydraulic, electrical and mechanical means and the like. FIG. 4 shows details of an example of such mechanism with a variable lift disc 7 and helical gear 8, both are used together to alter the rocker arm pivot 4 position. Under hydraulic pressure 15, the helical gear moves longitudinally and rotates the variable lift disc. It therefore alters the pivot of rocker arm assembly. The amount of disc rotation, rocker arm pivot motion, helical gear travelling is a function of hydraulic pressure, or engine speed.

[0039] FIG. 8 shows the details of third embodiment of the invention depicting the engaging teeth on outside diameter of the variable lift disc. The disc is not necessarily circular as illustrated, it can be a sector gear or with teeth on part of the disc, it can also be a lever or eccentric cam. As shown in FIG. 4, the mechanism of the variable lift disc 7 and helical gear 8, is similar, both are used together to alter the rocker arm pivot 4 position. Under hydraulic pressure 15, the helical gear moves longitudinally and rotates the variable lift disc from outside engaging teeth. It also achieves the object of altering the pivot of rocker arm assembly. The spring 10 or 11 is used to push back the helical gear back, thus changes the rocker arm pivot position back to low valve lift. The spring 10 and 11 can be replaced with a hydraulic chamber. The amount of disc rotation, rocker arm pivot motion, helical gear travelling is also a function of hydraulic pressure, or engine speed.

[0040] FIG. 9 and FIG. 10 are two more embodiments showing the driving mechanisms that alter the rocker arm pivot position when engaging teeth are located outside diameter of the disc.

[0041] FIG. 11 shows a normal valve lift diagram without any overlap in timing. Exhaust valve opens at bottom dead center (BDC) and closes at top dead center (TDC) after 180° cam angle, thereafter intake valve opens at TDC and closes at BDC after 180°. For simplification, the ramps at valve opening and closing are neglected in all lift diagrams form FIG. 9 to FIG. 16.

[0042] FIG. 12 is a conventional phasing diagram when the intake is advanced. Basically, it varies the valve timing by shifting the phase angle of camshafts. This movement is controlled by engine management system according to need, and actuated by hydraulic valve gears. Note that cam-phasing variable valve timing cannot vary the duration of valve opening. It just allows earlier or later valve opening. Earlier opening results in earlier close, of course. It also cannot vary the valve lift.

[0043] FIG. 13 is a conventional phasing diagram when the intake advanced and the exhaust retarded. This enables more overlapping, hence higher efficiency at high engine speeds, but maybe unstable at low speeds.

[0044] FIG. 14 is the continuously variable valve timing lift and duration diagram when the timing is set at the peak lift. At the peak lift setting, exhaust valve closes and intake valve opens at TDC when there is no lash in the lash compensator 13 due to the rocker arm pivot 4 movement. The intake opening is delayed and opening duration is shortened when the valve lift decreases.

[0045] FIG. 15 is the continuously variable valve timing lift and duration phasing diagram when the timing is set at the minimum lift. At the minimum lift setting, exhaust valve closes and intake valve opens at TDC when the lash is maximum in the lash compensator 13 due to the rocker arm pivot movement. The intake opening is advanced and duration is lengthened when the valve lift increases. The changes in timing, lift and duration can be continuous depending on engine speeds.

[0046] FIG. 16 is the continuously variable valve timing lift and duration phasing diagram when the intake advanced and the exhaust retarded. The setting is based on minimum lift, i.e., exhaust valve closes and intake valve opens at TDC when the lash is maximum in the lash compensator 13 due to the rocker arm pivot movement. Therefore, the overlap and valve lift duration increase when the valve lift increases. The change in timing, lift and duration is continuous and a function of engine speed. At low engine speed, such as at idle, valve lift is low and lift duration is short. Since the valve lift and duration control the air/charge flow, the result is that conventional throttling may be discarded, as valve motion alone may be enough to control the intake charge and the pump lost is then minimized. Valvetrain wear is also minimized, since at low lift and low engine speed, the valve seat seating velocity is low, the reciprocating travel and speed at stem/guide interface are low, and the contact stress between cam and cam follower is also low. There is a possibility that the hydraulic lash compensating device such as hydraulic lifter, lash adjuster and hydraulic capsule can be eliminated since the lash in the mechanical lash compensator 13 is now controlled by the rocker arm pivot movement in the continuously variable valve timing, lift and duration system. Another benefit of the system is, as in all variable valve timing devices, the increased overlap as the engine speed increases resulting in the internal exhaust gas recirculating (EGR) effect. The variable valve timing or increased overlapping can be achieved by the system without using the cam phasing device. Therefore, the continuously variable valve timing, lift and duration system can improve fuel economy, emission, engine performance, and valvetrain wear characteristics throughout the engine speed range without significantly compromising the design, structure, and cost.