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Page28 of 28GT-Suite Simulation Work Results

Unpublished addendumRev. 107-20-2011

A simple single-cylinder diesel engine, homologue to the test engine, has been simulated without and with added CGR (surcharging), based on an independent third party model and advises:

The simulation results support the test results. CGR appears to be an internal turbocharging with EGR.

Some plots will be inserted here for illustration. The executable GTM, GX and GU files are downloadable. These contain hundreds of plots, tables and animation files as well as all model attributes and results of runs.You need GT-ISE 7.0 and GT-POST 7.0 from Gamma Technologies to run the files. Due to the program constraints, the CGR valve was modeled split as added exhaust and intake valves. The engine with CGR is named Forward Flow and the one without, Blocked Flow. In the Forward Flow model, the gas flows forward from exhaust to intake through a common rail pipe, which is in essence a buffer vessel, because, in a multi cylinder engine, only one CGR valve opens into it at any one moment. In the Blocked Flow model, the CGR flow is blocked, by setting the CGR valve lift to zero. The two check valves abutting the CR pipe were modeled as simple Reed valves, which, under pressure, detain the CGR gas before reentry. The CGR valve openings overlap with the exhaust and the intake valves of the cylinder.

The model is configured to illustrate the great emission improving capability of the CGR at moderate performance gains. It shows that CGR is particularly beneficial at low engine speeds and loads. Its EGR does not reduce intake air mass and fades out at higher engine speeds without any control. It helps the engine to warm up quickly and to retain heat. It reduces peak combustion temperature and recycles heat. Apparently, one added double-lobe-valve per cylinder and a CR pipe/manifold can substitute a Turbocharger, an EGR system with a Charge Air Cooler (Intercooler), a Diesel Oxidation Catalytic Converter (DOC) and a NOx Storage Catalytic Converter (NSC), all without any controls. This model did not analyze soot and sulfuric emission components. The reader may expand on that with GT-ISE.From the actual gas analysis of the test engine with CGR however, one may reasonably expect that the Selective Oxidation Catalyst (SOC) and the H2S Catalytic Converter (DeSOx) may also be eliminated. If such eliminations however, contradict some other essential considerations, their complexity, size, cost and maintenance need may be reduced, nevertheless. In any case, it appears that the savings on the turbocharger or on the emission system alone would more than pay for the added CGR valve and pipe. The Diesel Particulate Filter (DPF) may have to be retained alas simplified. Note, that the experiments with and without CGR intercooler indicated small difference in engine performance and, that this simulation model does not include CGR intercooler here. Again, the reader may expand on that with GT-ISE.

One shall be aware that the extent of this substitute capacity is a given-and-taken in various extent, and that the real estate of the engine head is precious, and that a new valve addition requires a new cam lobe as well.

Some clearly labeled plots, with some explanations follow next, where Blocked Flow labels the stock engine without CGR and Forward Flow labels the same engine converted with added CGR:

CGR boosted indicated efficiency by 4% from 38.5% to 39.9%.

The CGR boosted volumetric efficiency of all gases by about 36% at low engine speed and 17% at high one.

The stock engine had no EGR, while the converted one had internal EGR, which declines with engine speed without any controls--that is automatically. Such EGR reduction by engine speed is desirable. In external EGR systems, it is commanded by controls at great expense.

Note, that the CGR (Combustion Gas Recycling) acronym was introduced by the author for mere convenience to distinguish internal EGR (Exhaust Gas recycling) from external one. In this website, it is used only under the Closing Report tab. The assertion was made that the cylinder gas is still in combustion before the blow-down upon exhaust valve opening. The distinction however may seem semantic. After all, residual combustibles oxidation takes place before and after the exhaust blow-down and the CGR gas pipe may be placed outside the cylinder head. Anyhow, the blow-down to the CGR pipe allows for further oxidation within the engine, inside the cylinder. That reduces power-losses and boost power concurrently. This type of "internal" EGR is also called in other documents CGR, surcharging, self-propelled EGR and by some other names and acronyms.

The CGR recycles some UBHC for multiple burning.

The CGR boosted the trapped gas mass by 26-37%at lower engine speeds and by 25-13% at higher ones.

On the same fuel injected, at lower engine speeds, the CGR reduced CO2 by 4%.

The CGR reduced HC concentration by 20-25%.

The CGR boosted intake pressure at the commencement of actual compression by 64% from 2.40 bar to 3.94 bar. That resulted in a 31% boost in peak cylinder gas pressure, which accounted for the torque and power boosts however. One may modify this model to verify that this CGR corresponds to 3.94-2.40=1.54 bar external charge boost by turbo-charging. One may notice that that would shift the loop down by 1.54 bar and eliminate the 31% pressure boost.

Excessive peak pressure may not be desirable, thus the CGR technology may better suit new engines, rather than engine retrofitting. Yet, on the same engine power, most diesel engines can be retrofitted for new emission regulation compliance. Old power generators appear to benefit most from CGR retrofitting for emission control.

The head of the exhaust flow is called the blow-down and the body of it, with the tail, is called the push-off or push-out. At the exhaust and intake valve opening overlap, some intake air escapes through the exhaust port and some exhaust gas remains entrapped, which is called internal residual EGR. The intake does not start with an impact-like head, but rather sinusoidal. This Blocked Flow gas cycle represent the classical diesel gas cycle or the stock engine here.

The CGR blows down first into the common rail pipe and thereafter into the exhaust pipe. Then, in the next cycle, the CGR gas bows down into the cylinder (blow-in). The shock waves, which follow the blow-in, contribute to EGR-air mixing. The CGR valve trims were 290 and 90 cam angles for the in-flow and ex-flow openings respectively, with half of the exhaust valve opening time and 60% of lift. During the real engine testing, sharper lobes proved to be wearing off prematurely and thus were replaced with normal lobes in accordance with this plot. Notice that the two openings of the CGR valve are almost in counter phase. That allows for the economical double-lobe CGR valve design. That also enables cylinder-per-cylinder CGR design, with or without common rail CGR pipe. This Forward Flow plot represents the surcharged gas cycle or the modified engine here.

The CGR boosted peak pressure by 31% from 125 bar to 164 bar. That contributed to engine performance boost.

The CGR reduced combustion temperature by 13% from 2,107K to 1,886K, which greatly contributed to NOx reduction. It also reduced exhaust gas temperature by 12% from 886K to 776K.

This CGR boosted turbulence 50 folds at BDC compression start. That ensured quick and efficient EGR mixing. That turbulence boost would greatly help to a spark ignition (SI) engine. However, inter-cooled CGR for SI engines has not been studied by GT simulations just yet. It is expected however, that knocking would seriously limit using CGR for petrol engines.

Note that this CGR reduced NOx concentration by 73% from 2,472 ppm to 667 ppm at combustion peak and at exhaust, by 97% from 388 ppm to 11 ppm.

The CGR has beneficial effects on the thermodynamics of the gas cycle.

The CO, NO and H2 emissions of the Blocked Flow engine are high.

The CGR reduced peak CO by 61% and peak H2 by 56%. It also reduced the exhaust NO by 95%. The reader may wish to open the downloadable GTM files to see these and other reductions by numbers. Running the model for longer time (over about a hundred more cycles) shall result in the disappearance of the discontinuities shown here at mid compression stroke, and in further reductions in harmful emission components.

Notice that, in the stock engine, the CO, NO and H2 gas components are passing increased (from full compression to full expansion). In the converted engine with CGR however, the NO, which is most harmful to health, is passing about ten folds reduced. From intake to exhaust, it is passing two folds reduced. No conventional EGR, or other emission control technique is capable for such a favorable trends and reduction levels.

This Blocked Flow plot represents the stock engine's O2, CO2 and H2Oemissions.

Both engines took in the same amount of O2, of which the stock engine used up 96% and the Engine with CGR 99.99%. The CGR boosted the oxidation rate at combustion 370 folds. On this same fuel consumption level, the CGR demonstrated less dramatic, but still beneficial effects on the CO2 and H2O contents. Notice that, what recycles here-taking off between 120 and 160 CA--is extremely or completely inert gas.

The CGR recycles--mainly in the compression stroke--and thus retains heat, thereby improves on engine efficiency.

Due to CGR, the volumetric efficiency builds up quickly.

At low engine speeds, the volumetric efficiency and the internal EGR build up in a few seconds.

Apart from the added CGR loop, the compared engines are identical. Testing was done similarly. The CGR conversion was made first (Forward Flow--in grey) and the CGR valve vas closed second (Blocked Flow--in yellow). The test four-cylinder engine had forward CGR flow from its inner two cylinders to its outer two cylinders. In conformance with this simulation results, some VCDS data posted under the Retesting Results tab support the claims and the explanations given in the reports herewith.

Average Pressure @ 2500 rpm (bar). Stock engine. The pressure in the CGR line is irrelevant, for its gas flow is blocked here (Blocked Flow engine). It is the entrapped gas pressure upon valve lock-down.

Average Pressure @ 2500 rpm (bar). The engine with CGR (Forward Flow engine) demonstrate higher pressure at the CGR return than at its feed.

Based on the average detention pressure at average engine speed, this CGR may be designated a 4.6 bar CGR system. That pressure may be controlled by the differential resistances of the two check valves. That was done on the test engine. To see that effect, the reader may wish to replace the simple check valves in the GX file with parametric controlled check valves.

The reader is encouraged to open the downloadable GU file to monitor the animation of the standing shock waves retained in the CGR pipes, which mix the internal EGR between consecutive cycles--that is before its reentry into the cylinder space. In part, that explains the great emission improvement due to CGR.

Further GT studies are underway. Results will be posted upon availability.

The test engine has been reconfigured with CGR flow and valve timing presented in this simulation work. The results of the corresponding retesting are posted under the Retesting Results tab.

The author wishes to thank Brad Tillock and EngSim, Co. for their contribution and valuable advice in this GT study.

July, 2011, Tempe, AZCopyright , 2019 LLC, 2011