In the Information Compilation thread, I was asked for more information on Eco Mode and MDS of the Hemi. Since there is a lot of information available on the MDS system, I am taking the time to break down the system and explain it is a way that is hopefully easy to understand. I hope this begins a series of great write-ups, explanations, and technical articles that benefit the viewers of this section. I will be linking this thread to the Information Compilation thread.
Let's start with an explanation of the goals of MDS, and what MDS actually stands for. MDS actually stands for Multi-Displacement Systems. What this means is that the number of cylinders used to power the vehicle changes when certain conditions are met. The goal of this is to decrease the fuel consumption of the engine while still offering as much power as possible.
Usually, engineers attempt to use less fuel and create more power by designing more efficient engines. MDS takes a different approach by looking at the various power needs of the vehicle. A good analogy of this would be programmable thermostats. Rather than just decreasing the temperature in your house from 72 degrees to 68, programmers allow you to keep the temperature at 72 when you are home and active, and decrease the temperature to 65 when you are asleep or at work.
The engine's technology has been recognized as one of the greatest engines for its year of production. The 5.7L Hemi has been recognized as one of Ward's 10 best engines 6 times from 2003 to 2009.
Despite what some might think (including myself prior to writing this), Chrysler is not the only manufacturer to offer cylinder deactivation technology. They were however the first manufacturer to offer the technology in North America on a large-volume scale. MDS debuted in 2004 for the 2005 model year. For the 2005 model year, it was only available on passenger cars. In 2006, recalibration of the system brought it to more trucks and SUVs.
The MDS system seamlessly alternates between smooth, high fuel economy four-cylinder mode when less power is needed, and V8 mode when more power from the is demanded. This change from one mode to the other in just 40 milliseconds (0.040 seconds). The use of an electronically controlled throttle is essential to this change. To make this change, the system deactivates the valve lifters. This keeps the valves in four cylinders closed, and therefore eliminates combustion.
In addition to stopping combustion, the system also eliminates the flow of air to the deactivated cylinders. This reduces power lost in the process of pumping in air and compressing it during the compression stroke. By eliminating the air flow to these cylinders, the system is also able to provide more air to the cylinders that are creating combustion. In large capacity engines (such as the Hemi), the pressure at top dead center (when the piston is the farthest from the crankcase) can drop dramatically and be very low. This low pressure results in poor efficiency. Since half of the cylinders are not drawing in air under these light load conditions, the engine is able to be more efficient.
Changes in the flow of air is not limited to the intake. In order to preserve the characteristic rumble of the V8 engines, Chrysler designed a special exhaust system for MDS equipped vehicles. This includes four separate mufflers, two large central ones for V8 mode and two smaller ones near the tailpipes for four cylinder mode.
Design and Components
The MDS provides cylinder deactivation during steady speed, low acceleration and shallow grade climbing conditions to increase fuel economy. Both four and eight cylinder configurations have even firing intervals provide smooth operation. Two cylinders on each bank are active when the engine is in four-cylinder mode, every other cylinder in the firing order. All of the cylinders that are deactivated have unique hydraulic valve lifters that collapse when deactivated to prevent the valves from opening. Engine oil pressure is used to activate and deactivate the valves. It is delivered through special oil passages drilled into the cylinder block. Solenoid valves control the flow. When activated, pressurized oil pushes a latching pin on each valve lifter, which then becomes a “lost motion” link. Its base follows the camshaft, but its top remains stationary, held in place against the pushrod by light spring pressure but unable to move because of the much higher force of the valve spring. Deactivation occurs during the compression stroke of each cylinder, after air and fuel enter the cylinder. Ignition then occurs, but the combustion products remain trapped in the cylinder under high pressure, because the valves no longer open. No air enters or leaves. During subsequent piston strokes, this high-pressure gas is repeatedly compressed and expanded like an air spring, but fuel is not injected. The cylinders deactivated are 1,4,6, and 7. The normal firing order is 1-8-4-3-6-5-7-2.
The PCM will activate the 4-cylinder mode when the following enabling conditions are met:
* Low engine load
* 1200 to 3000 rpm
* Vehicle speed between 12 mph and 90 mph
* Battery voltage from 9 volts to 15 volts
* Oil pressure from 15 psi to 147 psi
* Oil temperature greater than 120 F
* Engine coolant temperature from 158 to 248 F
* Ambient temperature from 14 F to 248 F
* No throttle position sensor or MDS faults
* Trap an exhaust charge
* Normal combustion event
* Don’t open exhaust valve
* Don’t open intake valve
* Piston is an air spring
* Cylinders deactivated in firing sequence
* Empty the cylinder
* Open exhaust valve
* Open intake valve
* Normal combustion event
* Cylinders reactivated in firing sequence
* Unique MDS camshaft.
* Deactivating roller tappets.
* 4 control valves/solenoids.
* control valve/solenoid wiring harness.
* oil temp sensor.
Note: Use of 5W-20 engine oil is critical to the operation of the MDS. Thicker oil clogs the oil passages and could lock cylinders into being perpetually closed or open.
Duty Cycle Graph
MDS was engaged for about 17% of city driving and 48% highway for an overall average of 40%
Tips for Maximizing Efficiency
There are a few simple that that you can do to improve the efficiency of your driving when driving a vehicle equipped with MDS:
* Keeping speeds to 65 mph or below – MDS uses four cylinder mode most at these speeds
* Use cruise control – this helps maintain a steady speed, generally allowing the HEMI to run on four cylinders for longer periods
* Accelerate more gradually – the HEMI will provide V8 power whenever it is requested by the driver
* Use a steady throttle whenever possible – this maximizes four cylinder mode
I hope people find this information useful and informative.