Rover Mini Specifications download pdf (Page 4)

14-4 Rover MEMS - MPi/SPi

Amplifier

The MEMS amplifier contains the circuitry

for switching the coil negative terminal at the

correct moment to instigate ignition. The

signal received by the amplifier from the CAS

trigger is of an insufficient level to complete

the necessary coil switching. The signal is

thus amplified to a level capable of switching

the coil negative terminal.

The amplifier circuitry is contained within

the ECM itself, and the microprocessor

controls the ignition dwell period for each

condition of engine speed and battery

voltage.

Dwell operation in MEMS is based upon the

principle of the 'constant-energy current-

limiting' system. This means that the dwell

period remains constant at about 3.0 to

3.5 ms, at virtually all engine running speeds.

However, the dwell duty cycle, when

measured in percent or degrees, will vary as

the engine speed varies.

Ignition coil

The ignition coil utilises low primary

resistance in order to increase primary current

and primary energy. The amplifier limits the

primary current to around 8 amps, and this

permits a reserve of energy to maintain the

required spark burn time (duration). In DIS

systems, the coils are double-ended, and fire

two spark plugs together. The KR6 utilises

three DIScoils, and the MGF two DIS coils.

Distributor

In the MEMS system, the distributor only

serves to distribute the HT current from the

coil secondary terminal to each spark plug in

firing order. The distributor is located on the

inlet camshaft at the cylinder No 4 end. The

distributor contains a rotor arm, and also has

a deflector plate and oil drain to prevent oil

seal leakage from contaminating the

distributor cap and rotor arm.

Distributorless

ignitionsystem (DIS)

Vehicles with the KR6V6 engine, and those

with the four-cylinder MGF WC engine utilise

wasted spark DIS ignition. The MGF without

WC is equipped with a distributor. Refer to

Chapter 2 for a detailed description of wasted

spark and DIS.

Knock sensor

(some MPi vehicles)

The optimal ignition timing (at engine

speeds greater than idle) for a given high-

compression engine is quite close to the point

of onset of knock. However, running so close

to the point of knock occurrence means that

knock will certainly occur on one or more

cylinders at certain times during the engine

operating cycle.

Since knock may occur at a different

moment in each individual cylinder, MEMS

employs a knock control processor (KCP)

built into the ECM to pinpoint the actual

loo

cylinder or cylinders that are knocking. The

knock sensor is mounted on the engine block,

and consists of a piezo-ceramic measuring

element that responds to engine noise

oscillations. This signal is converted to a

voltage signal that is proportional to the level

of knock, and returned to the ECM for

evaluation and action.

The ECM will analyse the noise from each

individual cylinder, and uses a sophisticated

technique to recognise knock as distinct to

general engine noise.

Initially, timing will occur at its optimal

ignition point. Once knock is identified, the

microprocessor retards the ignition timing for

that cylinder in steps of 0.625° until either

knock ceases or a maximum retard of 10° is

reached. The timing is then advanced in

0.65° increments until the reference timing

value is achieved or knock occurs again,

when the processor will retard the timing once

more. This procedure continually occurs so

that all cylinders will consistently run at their

optimum timing.

If a fault exists in the KCP, knock control

sensor or wiring, an appropriate code will be

logged in the self-diagnostic unit, and the

ignition timing retarded by 10.5° by the LOS

program.

5 Fuelinjection

Rover has adopted three distinct methods

for providing fuel to the engines equipped

with MEMS. The methods are simultaneous

multi-point injection (MPi), sequential multi-

point injection (MPi) and single-point injection

(SPi).

Because of the modularity of MEMS, very

little difference exists between the implemen-

tation of each system on the various engines.

First, a description of common features and a

description of each type follows.

The injector(s) are switched using two

circuits. Operation depends on the principle

that more current is required to open an

injector than to keep it open. This kind of

system is often termed 'current-controlled'.

Once the injector is open, a second circuit

rapidly pulses the injector to earth. The

switching is so rapid that the injector is

effectively held open, and less current is

required during the operation. Advantages of

this arrangement include a reduction in

injector operating temperature, and

immediate injector closure once the holding

circuit is switched off.

The MEMS ECM contains a fuel map with

an injector opening time for basic conditions

of speed and load. Information is then

gathered from engine sensors such as the

MAP sensor, CAS, CTS, ATS and TPS. As a

result of this information, the ECM will look up

the correct injector pulse duration right across

the engine rpm, load and temperature range.

The fuel injector is a magnetically-operated

solenoid valve that is actuated by the ECM.

Voltage to the injectors is applied from the

fuel pump relay, and the earth path is

completed by the ECM for a period of time

(called pulse duration) of between 1.5 and

10 milliseconds. The pulse duration is very

much dependent upon engine temperature,

load, speed and operating conditions. When

the magnetic solenoid closes, a back-EMF

voltage of up to 60 volts is initiated.

The amount of fuel delivered by the

injector(s) is determined by the fuel pressure

and the injector opening time - otherwise

known as the pulse duration. The ECM

controls the period of time that the injector is

held open, and this is determined by the

signals from the various sensor inputs. During

engine start-up from cold, the pulse duration

and number of pulses (frequency) are

increased to provide a richer air/fuel mixture.

Over-speed fuel cut-off

(rev limiter)

. To prevent over-high engine speeds, which

might otherwise lead to engine damage,

above 6250 rpm (MPi) and 6860 rpm (SPi),

MEMS inhibits the injector earth path. As the

engine speed drops below 6150 rpm and

6820 rpm respectively, fuel injection is

reinstated.

Deceleration fuel cut-off

A deceleration fuel cut-off is implemented

during engine over-run conditions, to improve

economy and reduce emissions. The

conditions for over-run to be implemented

are:

a) Throttle closed (throttle pedal contacts

closed).

b) Engine speed above 2600 rpm (MPij or

1500 rpm (SPij.

c) Coolant temperature above BO°C.

d) Once the engine speed drops below

2600 rpm or 1500 rpm respectively, fuel

injection is reinstated.

Multi-point injection

(MPi

-simultaneous)

The MPi system consists of one injector for

each cylinder, mounted in the inlet port, so

that a finely-atomised fuel spray is directed

onto the back of each valve. The injectors are

all pulsed simultaneously, twice per engine

cycle. Half of the required fuel per engine

cycle is injected at each engine revolution.

Fuel will briefly rest upon the back of a

valve before being drawn into a cylinder.

Unlike other simultaneous systems, the

injectors are all connected to the ECM via

separate wires to separate ECM driver pins.

Multi-point injection

(MPi

-sequential)

The sequential system functions in a similar

manner to the simultaneous system.

However, with reference to the signal from the

cylinder identification (CID) sensor (only

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Rover Mini Specifications Page 4

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