Engine Talk

[mts]

Mazda/Eunos KJ-ZEM

A much underrated engine is the Mazda/Eunos 800 Miller-cycle KJ-ZEM.

 

 

Sold in Japan from 1997, the 2.3 litre V6 KL-ZEM employs DOHC, 4-valve-per-cylinder heads with Miller cycle technology. The Miller-cycle principle involves closing the intake valves much later than in a conventional Otto cycle engine (the inlet valves are kept open for the first 20 percent of the compression stroke). This approach reduces pumping losses at part throttle. A Lysholm screw-type supercharger is an essential part of the system because it prevents the mixture flowing backwards out through the inlet valves (which are held open). Twin-air-to-air intercoolers are also fitted.

 

With a static compression ratio of 8:1 and boost pressure of around 14 psi, this sophisticated engine produces 162kW at 5500 rpm. Peak torque (294Nm) is available at 3200 rpm. Interestingly, the Japanese-spec engine is 13kW and 12Nm gruntier than the Australian-delivered version. All KL-ZEMs are fitted with a 4-speed automatic transmission.

 

 

We are told that the engine is reliable but the hard-working supercharger can develop seal problems. A replacement blower costs a few hundred dollars.

 

KL-ZEM on the Dyno

 

 

In our test of the Mazda 800 back in 1999 (Eunos 800 Miller cycle) we decided to run the supercharged Miller-cycle engine on a Dyno Dynamics chassis dyno. As seen in this graph, the Australian-spec (149kW) engine produced 111kW at the wheels with strong low-to-mid rpm torque.

 

 

Interestingly, intake air temperature shot through the roof during testing after about 10 seconds running at full power, we measured 75 degrees Celsius after the little intercoolers (see pic)! Whack on some better intercoolers and you're sure to have more power.

 

[mts]

 

The Lysholm compressor developed by Mazda and IHI are pulled by a belt notched since the crankshaft. It contains, like Roots, two parallel rotors but those one in another lobes in the shape of helical slope encasing itself according to a rather complex kinematics. The convex rotor "male" is equipped with three blades and its maximum speed is of 35.000 rpm whereas the concave rotor "female" with five blades turns to 21.000 rpm. Helical pinions ensure their synchronization. Aspired at an end, the air is compressed by the lobes which make it circulate axially to its exit towards the exchangers. The choice of this type of compressor was guided by its pressure of overfeeding appreciably higher than that of a Roots blower all the more necessary as it should overcome the flow of gases in phase of increase of the piston the admission being open. In addition, because of its weak impulses, the emission of noise of the Lysholm compressor are lower than those of Roots.

 

[mts]

It's not all torque

The inside story on the Miller-cycle engine

 

The name for the Miller-cycle engine comes from an American engineer, Mr Ralph Miller who patented his version of the forced induction Otto-cycle in the 1940s.

 

Until now his principle had only been used in low engine speed applications - such as driving big ships and also for power generation by stationary engines.

 

The engine in the Mazda Millenia is a 2.3 litre, quad cam V6 which is designed to perform better than a larger 3.0 litre engine but with the efficiency of a smaller (2.0L) unit.

 

It provides the driver with high performance coupled with between 10 and 15 percent less fuel consumption.

 

Power and torque figures are: 164kW of power @ 5.500rpm and 294Nm of torque @3,500 rpm. This compares with 149kW @6500 and 223Nm@4800 for the base 2.5 litre vehicle equipped with the conventional Otto-cycle engine.

 

From the outside, the Miller engine looks similar to other hi-tech units. Aluminium block, lots of belts, 24 valves, four camshafts, except for the two intercoolers and a belt driven Lysholm compressor tucked neatly into the "Vee" between the cylinder banks.

 

So, how does this 2.3 litre engine produce more power and torque using less fuel than a larger engine, without many of the expected disadvantages; such as high emissions and engine knock?

 

 

In simple terms, the compression stroke of the Miller-cycle engine is shortened with results in a low compression ration, yet a high expansion ratio.

 

In order to grasp this and other aspects of the Miller-cycle, one has to go back and have a look at some of the basic principles of internal combustion engine operation. There are four areas worth reviewing.

 

Engine Size vs Frictional Losses

 

When the displacement of an engine is reduced, there is a substantial reduction in frictional losses. For example, 25 percent less friction is produced rotating a particular engine that has its displacement reduced by 30 percent. An automatic offshoot of such downsizing is an improvement in fuel efficiency of around 10-15 percent.

 

Theoretical vs Actual Compression Ratio

 

The theoretical compression ratio is simply a comparison of the volume above the piston when it is at bottom dead centre (BDC), to the volume above it at top dead centre (TDC). However, in practice, the actual compression ratio is determined by the valve timing, since the real compression stroke does not begin until the intake valve closes. Similarly, the length of the power (expansion) stroke is also determined by the opening point of the exhaust valve.

 

With the fairly symmetrical valve timing being found in most engines these days, these two strokes are approximately the same. This means that the actual compression stroke is roughly equal to the expansion stroke.

 

Thermal Efficiency

 

By increasing the compression ratio, the thermal efficiency of an engine is also increased. However, along with this efficiency gain comes higher combustion pressures and temperatures. These characteristics are usually accompanied by two well known "bad guys" Oxide of Nitrogen (NOx) emissions and knock.

 

NOx is produced as a result of combustion pressures and temperatures greater than 1,300 deg C. At these temperatures the normally inert Nitrogen (78 percent by volume of intake air), reacts with oxygen to form oxides (nitrogen dioxide and nitrogen monoxide).

 

Knock is caused when part of the air/fuel charge is ignited spontaneously by the effect of heat and pressure and not the spark plug as Otto intended. This produces two flame fronts in the combustion chamber which can result in serious engine damage.

 

There are two important things to note here. Firstly, knock is affected by the gas temperature at TDC of the compression stroke. Secondly, most of the gain in thermal efficiency from increases in compression comes mainly from the events that occur on the expansion stroke (more push on the piston). Only a little is gained from the actual increase in compression ratio.

 

Pumping Losses

 

This refers to the energy required to rotate an engine during two of the three non-power producing strokes

 

- pumping air in and pumping exhaust gases out (but does not include frictional losses). It is a term that describes the efficiency of intaking and exhausting the charge. If the piston does less work in taking and exhausting, less power robbing pumping losses are produced.

 

One of the reason the original Otto-cycle had the exhaust valve opening brought forward (before BDC) is to allow the residual exhaust gas pressure (which, once the piston is half way down the power stroke is too low to provide much push on the piston) to expel itself and not have to rely on the piston to pump it all out, creating further pumping loss. This modified (Otto) valve timing allows around 50 percent of the exhaust gases to be expelled "for free" (no pumping losses incurred in getting rid of half of the exhaust gas). A throttled engine (eg cruising with high manifold vacuum) has high pumping losses since a vacuum is not produced for free; energy is consumed in doing so. Some experimental variable displacement engines reduce the number of working cylinders (switching some off by holding the valves open) under partload to reduce manifold vacuum and therefore pumping losses.

 

Volumetric Efficiency

 

The term volumetric efficiency refers to the ability of an engine to fill its cylinders with a volume of air equal to their displacement (100 percent Ve). The greater the Ve then the greater will be the output of that engine. Engine manufacturers go to great lengths to "tune" their engine design and obtain the greatest Ve. This involves a lot of research into gas flow - including manifold and port design - as well as valve timing and lift, together with multiple valves and combustion chamber design.

 

The easiest way to make dramatic improvements in Ve is to add an external device such as a supercharger or turbocharger. Its job is to "force feed" as much air as possible into each cylinder. But, as with increased compression ratio, excessively high combustion pressures and temperatures may be produced by forced induction. These can work against our intent to produce a powerful but clean engine.

 

The most common method to overcome this problem is to use an intercooler (as well as lowering the compression ratio). An intercooler is an air-to-air heat exchanger that has the ability to reduce air intake temperature (after the supercharger) by at least 50 deg C. This helps keep combustion temperatures to a safe level.

 

The modern internal combustion engine is a finely balanced mixture of all these (and many more) conflicting requirements.

 

Miller-cycle Technical Details

 

There are basically four means that the Miller-cycle uses to obtain its increased efficiency.

• Smaller engine (lower displacement)
• reduced compression stroke and pumping losses

(from late closing of the intake valve)

• cooler intake charge (intercooled air)
• combustion improvements

Small Engine

 

The graph below indicates the fuel efficiency increase as displacement is decreased. The horizontal axis begins at 1.0 which compares to a 3.3L's fuel efficiency, whilst 0.7 indicates a 30 percent reduction in displacement (down to 2.3L). The two curves represent the changes in efficiency gain with load changes (the greatest being at 20 percent load).

 

 

An engine that has a lower compression ratio will also naturally produce smaller amounts of friction, particularly on the compression stroke. Since the Miler engine is targeted at a vehicle that would normally use an engine over 3.0L, the reduction in size to 2.3L provides an improvement in fuel efficiency of around 13 percent.

 

Reduced Compression Stroke Retaining High Expansion Stroke

 

At first glance the compression ratio would appear to be 10:1 (swept volume compared to clearance volume), however, for the first 20 percent of the compression stroke, the intake valves remain open. Since the actual compression stroke does not begin until the valve closes, the compression ratio is "artificially" reduced down to 8:1.

 

 

Intake valve duration is from two degrees before TDC until 70 degrees after BDC, while the exhaust valve duration is from 47 degrees before BDC to five degrees after TDC. The intake valves remain open for around an additional 30 degrees of crankshaft rotation beyond "normal". This kind of valve timing reduces the effective compression ratio from 10:1 to a little under 8:1.

 

Unusual is the fact that the compression stroke is reduced but the power or expansion stroke remains the same. This is one of the critical points of difference from the Otto-cycle engine where the relationship between the expansion and compression is the same.

 

The late closing of the intake valve eliminates the substantial amount of energy normally required to overcome friction (as well as pumping losses), in the process of completing a normal compression stroke.

 

While this sounds good in theory, the usual result of blowing half the intake charge back out the intake valves would be a reduction in volumetric efficiency.

 

In the Miller-cycle engine, however, this is where the compressor comes to the rescue. Any loss of intake charge through "back flow" is more than compensated for by the density of the charge provided by the compressor. Under these circumstances, the Lysholm compressor is more efficient (lower pumping loss) at carrying out the job of filling the cylinders than a reciprocating piston.

 

The highly efficient Lysholm compressor consists of a male and female rotor, with three and five lobes respectively. Rotor speeds are up to 35,000rpm for the male and 21,000rpm for the female. Maximum discharge pressure is up to 150kPA. Advantages of the belt driven compressor include no lag, non-contacting rotors and none of the temperature extremes associated with turbocharger operation.

 

Cool Intake Charge

 

Due to the late closing of the intake valves (reduced compression ratio), less heat is added to the intake charge by the piston during this stroke. The loss in thermal efficiency of reduced compression ratio from 10 to 8:1 is only about six percent.

 

This slight loss in thermal efficiency from the decrease in compression ratio is more than made up for by a much denser charge supplied by the compressor. Cool dense air is pushed through twin intercoolers into the cylinders. This reduces the combustion chamber temperature at TDC of the compression stroke and so lowers the potential for detonation to occur and also production of NOx.

 

The end result of this delicate balance of valve timing, compression ratio and forced induction, is a cylinder that is well filled with cool dense air but at a lower cost in terms of energy consumption than a conventional four cycle would allow.

 

Combustion Improvements

 

For many years, swirl and squish were commonly used terms to describe the in-cylinder events affecting the rate and other characteristics of combustion. In more recent years, extensive study of vertical in-cylinder swirl, called "tumble" has been carried out.

 

On the Miller engine, the intake port has been shortened to promote smooth but strong intake air flow. A mask is added to the intake side of the combustion chamber to concentrate the air flow to the centre of the cylinder; strengthening the tumble motion.

 

Tumble promotes more ideal intake dynamics and combustion events that enhances the anti-knocking performance of the engine.

 

Conclusion

 

This engine utilises well proven conventional technology, but further enhances it to take into account growing international concerns for the environment and resource preservation.

 

While, in the fullness of time, engines which use alternative forms of energy may come to pass, Miller-cycle technology will be seen to have advanced the cause of efficiency and responsibility.

[Renboy]

I told you bitches that the compression was effectively 8:1 (a little under 8:1 to be exact). Premium gas my fucking ass. And where in the fuck are they getting the s/c's for a few hundred dollars?

 

Nice find btw.

[mts]

I laughed at that too.... couple thousand maybe

[enginph]

What dolar is that?

 

For the compression. I remember we talked about it before. The important thing for self ignition is the absolute pressure in the cylinder when its in TDC (where pressure of unburned fuel is max). For naturally aspired engines compression ratio can be used as a measure for self ignition since it will scale the atmospheric pressure to that max pressure. But for miller,we have the s/c that increases the pressure! So you cannot just look at the compression ratio and decide what octane to use..

 

However since the gas prices are crazy, i put mid grade nowadays Forget about comp ratio this is a better motivation to use lower octane for me..

 

 

I laughed at that too.... couple thousand maybe

 

[mts]

It runs the mid-grade without a problem so far... I still have to get an emissions check done at the lower grade. BUT there is NO noticeable difference in the performance of the engine, maybe even a slightly better throttle response at mid grade.

[ing-schu@online.no]

 

The nominal compression, as in volume decrease from BDC to TDC is 8:1.

 

However the SC charge supplies precompressed air, even on idle speed, resulting in an effective compression ratio of 10:1.

[mts]

Which still results in a lower compression ratio than most engines that require Premium

[ing-schu@online.no]

My BSA is 4,9:1...

[enginph]

Not that simply. The worst case is WOT. S/C pressure is 13PSI as a member with s/c pressure gause reported IIRC. That s/c increases the temperature as well! and there are the i/c s.. Not straightforward. I am sure it will knock without i/c at WOT...

 

 

The nominal compression, as in volume decrease from BDC to TDC is 8:1.

 

However the SC charge supplies precompressed air, even on idle speed, resulting in an effective compression ratio of 10:1.

 

[Renboy]

A stock KLDE engine can be run safely at 8 psi without intercoolers. The engine has a 9.2:1 compression ratio. I don't see how 7.9:1 with intercoolers and 13psi is a problem.

 

What's a BSA, and you could run that thing on pure septane without knock.

[mohamedeladawy@hotmail.com]

how about a rotary turbo? i put premium in it cuz well...ive always heard its always good to put premium in forced induced cars (excluding millenia.)

[ing-schu@online.no]

A BSA (Birmingham Small Arms) is (this one, they made rifles and bicycles too) a motorcycle. She pumps out a full 13 hp on 4200 rpm from her 500 cc displasement, and can do 60 mph on a good, cool day. She is from 1944 and I like her a lot. Imigine the smooth idle in a 4,9: 1 huge flywheeled engine, she has been known to run steady on 360 rpm. Thump-thump-thump

 

[attachmentid=611]

[Renboy]

A BSA (Birmingham Small Arms) is (this one, they made rifles and bicycles too) a motorcycle. She pumps out a full 13 hp on 4200 rpm from her 500 cc displasement, and can do 60 mph on a good, cool day. She is from 1944 and I like her a lot. Imigine the smooth idle in a 4,9: 1 huge flywheeled engine, she has been known to run steady on 360 rpm. Thump-thump-thump

 

[attachmentid=611]

 

 

And camel, I don't know enough about the rotary. For example, what the weakest link in the engine is. Given the design of the engine, I don't see why you couldn't plough a lot of boost into it. There's no piston, no rings, and no rods. Not to mention, the combustion 'chamber' in those engines is really long, and it's compression ratio is small ~8.5:1 iirc. To me, this sounds like the type of engine you plough 20lbs of boost into and scream down the street.

[mts]

The weakest link that holds up the forced induction of the rotary engines is the seals, which also are the only reason why there is a redline.

[Frankzao]

The gears and seals are theweekness

[mts]

Although in two dimensions the seal system of a Wankel looks to be even simpler than that of a corresponding multi-cylinder piston engine, in three dimensions the reverse is the case. As well as the rotor apex seals evident in the conceptual diagram, the rotor must also seal against the chamber ends.

 

Piston rings are not perfect seals. Each has a gap in fact to allow for expansion. Moreover the sealing at the Wankel apexes is less critical, as leakage is between adjacent chambers on adjacent strokes of the cycle, rather than to the crankcase. However, the less effective sealing of the Wankel is one factor reducing its efficiency, and confining its success mainly to applications such as racing engines and sports vehicles where neither efficiency nor long engine life are major considerations. Wankel engines should never be started and run unless the engine will reach operating temperature; most such instances of jammed engines occur when a car is started and moved a few yards, e.g. from a garage to a driveway. In these situations it is better to push the car and not start the engine. This is due to the engine flooding with fuel and essentially "Hydrolocking" the motor. This "Flooding" is caused by the excess amount of fuel injected into the engine in its "cold" running circuit.

 

50% longer stroke duration, as a piston engine (Wankel engine).

 

← The Quasiturbine has similar disadvantages with its concave combustion chamber, and in the AC design the sharp angles of the carriers hamper the propagation of the flame front, leading to incomplete combustion. The stroke duration is too short for a complete combustion.

 

 

[enginph]

How do they start it then if you shouldnt "Wankel engines should never be started and run unless the engine will reach operating temperature"

 

[mts]

It is saying they are not designed for short run times... like running out to the market and shutting it down before allowing it to reach operating temps...

[Renboy]

How do they start it then if you shouldnt "Wankel engines should never be started and run unless the engine will reach operating temperature"

"Wankel engines should never be started and run unless the engine will reach operating temperature"

 

If you don't plan on bringing it to temp, don't start it. With this hinderance I wonder if all of them came from the factory with block heaters? If not, they fucking should have.