Filed under: LA Auto Show, Tech, Acura
In depth: Acura's 2.3L turbocharged I-4 engine
When Acura designed the RDX, they wanted to give it a lightweight mill that was powerful enough to compete in a class rife with six-cylinder power. A V6 would provide the desired output, but it would add too much mass over the front axle, so instead, Acura decided to slap a turbo onto a four-cylinder and boost output accordingly. Honda, Acura's parent company, has developed some of the best performing four bangers in terms of horsepower-per-liter, so the engineers had a lot of innovation at their fingertips. The package put together for the RDX is a 2.3-liter inline-four, complete with Honda's i-VTEC variable valve timing and creating 240 hp at 6000 rpms and 260 ft-lbs of torque at 4500 rpm. Thanks to the addition of the turbocharger, it makes more torque than any other engine in Acura's lineup -- not a huge feat considering that most of Honda/Acura engines are devoid of any low-end grunt. The engine also features some economical innovations to keep the RDX's price tag from skyrocketing, so when we found this engine on display at the Acura booth at the 2007 Los Angeles Auto Show, we thought a closer look was in order. Follow the jump for an in-depth view with pictures from the display engine and Acura's own renderings.
Using a small four-cylinder, as opposed to a V6 engine configuration, has a significant advantage when it comes to meeting emissions standards. The RDX mill meets CARB LEV II ULEV (ultra low emissions vehicle) along with EPA Tier-2 Bin-5 standards. The reduction of hydrocarbons is achieved by placing the catalytic converter as close as possible to the turbine outlet of the turbocharger. This enables the cat to heat up quickly so it can do its job of converting hydrocarbons and carbon monoxide into carbon dioxide and water, along with breaking up NOx. The engine's cylinder head is also designed with internal exhaust gas recirculation. This significantly reduces the occurrence of NOx emissions by lowering temperatures in the combustion chamber that lead to the generation of nitrogen oxide and nitrogen dioxide.
Certainly the most impressive development of Acura's 2.3L engine is the design of the turbocharger turbine manifold. The manifold has a variable geometry arrangement that doesn't involve a lot of complication. This design has similar benefits to the variable-vane turbocharger used by Porsche, but not as costly or intricate to implement. Variable turbine geometry allows the turbocharger to perform over a wider range of exhaust flow rates. As engine rpm increases so does the air flow rate through the engine and out the exhaust. If a turbocharger turbine manifold is sized to flow for maximum horsepower, then the engine must reach a certain level of rpms before enough exhaust gas is circulating through the manifold for its energy to be effectively recovered by the turbine wheel of the turbocharger. By reducing the area ratio (A/R) of the turbine manifold, the turbine wheel can begin to effectively recover exhaust gas energy at lower rpms. However, the smaller size manifold begins to restrict the flow as rpms increase. This in-turn limits the amount of power the engine can produce. To solve the problem Acura created a variable flow turbine manifold that has multiple flow paths for exhaust gas. 
Acura's variable flow turbine manifold works much like a traditional twin-scroll turbine housing. A twin-scroll turbine housing has a divider in the center of the exhaust flow path. It works with exhaust pulses by keeping the exhaust flow of consecutive cylinders separate in order to capture more of the exhaust gas energy. However, Acura utilized this design differently. They created a diaphragm actuator-controlled trap door that restricts the exhaust gas to one scroll at lower rpms, then flips away to allow the gas to flow through both scrolls at higher rpms. It can be seen as the red painted flap on the display engine. This design allows the small engine to make use of its entire exhaust flow at low rpms, as opposed to a traditional twin-scroll design that only feeds half the exhaust through each scroll. Other elements of the engine design can be seen in the cutaway photos, such as the top mount intercooler. Be sure to visit the gallery for a host of high-res images to get a better look.
Reader Comments (Page 1 of 4)
paul34 5:17PM (11/22/2007)
>> not a huge feat considering that most of Honda/Acura engines are devoid of any low-end grunt
1) You're going to get a lot of flak from the honda guys for this comment. I'm just warning you.
2) Though I do not own a Honda, as a valet, I do have the opportunity to drive many. As such, I will say that though they produce many engines indeed lacking in low-end torque, many of their newer engines - such as the one found on both the I4 and V6 Accord - certainly have sufficient torque in the low-end. It's pretty surprising, actually.
On a related note, not all Honda engines have stratospheric redlines, either. The forementioned Accord engines have redlines comparable with most other mass-market engines. I don't feel like looking up the exact number, but I believe, if I remember correctly, it's something like 6500-ish (judging from the tach).
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SHR 5:34PM (11/22/2007)
I'm a bit confused as to is this something special because it's a Honda or what? Certainly the power output is nothing special. For example, the Mazdaspeed 3 has a Type-L3T 2.3L Direct Injection Spark Ignition (DISI) Turbocharged 16-valve 4-cylinder with 263 @ 5500 RPM, torque (lb-ft) 280 @ 3000 RPM with a Redline @6500. I'm sure there are other 4 cyl engines with better power/torque, so it seems Honda is just beginning to play catch up.
Still, glad conservative Honda (Acura) is finally getting some major grunt to go with their small 4-cyl cars. About time.
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EMC 5:43PM (11/22/2007)
its special because they are looking at turbocharging, when they have almost totaly ignored it in favour of "all motor" engines in the past.
It is interesting to note the increase in popularity with turbochargers recently. ;)
Temple 10:09PM (11/22/2007)
what's special?
The Acura RDX is the first mass-production gasoline engine to get a modern iteration of the Variable Geometry Turbo. The Porsche 997 Turbo is the second. VGTs are common in diesel engines, it allows the turbo geometry to change for varying boost pressure, the high temperature of the exhaust made gasoline engines much less reliable and unfit for the mass-production. the 1989 Shelby CSX-VNT was the first, but under a much smaller scale at only 500 produced.
rgseidl 12:07PM (11/23/2007)
Fully variable turbine geometries (VTG) - such as those used on many modern diesels as well as the Porsche 997 turbo - use a mechanism that can adjust the angle of a set of vanes to an arbitrary angle in a given range. The high engine-out exhaust temperatures of gasoline engines at rated load mean you have to use exotic alloys to achieve the required longevity.
Honda has decided to apply the much simpler variable slider turbine (VST), which supports only two discrete settings but can be produced at much lower cost. When applied to an inline 4, the exhaust from all four cylinders is routed through just one scroll in part load operation. The downside is crosstalk in the exhaust manifold: each cylinder's exhaust valves will open before those of the preceding one are fully closed.
This inadvertently recirculates some hot exhaust gas back into that preceding cylinder, increasing initial temperature there - exactly what you don't want in conventional combustion, because you have to retard the ignition to avoid engine knock. This results in poor fuel economy in part load, which is where regular - as opposed to sports - car engines spend most of their working lives.
Exhaust manifold crosstalk does not occur if any given turbo scroll is limited to a maximum of three cylinders with equidistant firing sequence (i.e. at least 240 degrees crankshaft in a four-stroke engine). Daimler does use a fixed-geometry single-scroll turbo in its sequentially charged inline 4 DiesOtto concept, but that also implements HCCI combustion in part load. In that context, hot EGR is actually required to sustain operation.
Gary Blomquist 1:00PM (11/23/2007)
I don't think many small displacement I-4 turboed engines can hold a candle to the 2.3 Mazdaspeed engine, and overall vehicle package. Just go to Youtube and type in Mazdaspeed3, and see how many video clips there are with stock MZ3's eating up cars with allegedly better credentials.
The price of a mazdaspeed3 in insanely low, compared to the competition. Talk about bang for the buck.
rgseidl 5:50PM (11/23/2007)
@Gary Blomquist -
yeah, the Mazdaspeed3 has 263 hp and 3153 lbs curb weight against the Acura's 240hp and gadget-bloated ~3930lbs.
Note that both are turbocharged and both feature AWD, which is a good idea at these power levels. Mazda's engine features DI, though. On a track, the Mazda should indeed smoke the RDX. The RWD, also turbocharged Solstice GXP might be a tad faster, but it's only a two-seater roadster.
Riceman 5:37PM (11/22/2007)
no flak at all. As a big Honda fan I freely admit and lament that iVtec are horrible at low end torque. My gf's dodge minivan has more low end torque than my civic which is a much lighter vehicle.
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psarhjinian 5:48PM (11/22/2007)
Your Dodge minivan has a much bigger engine. Compare the Caliber to the Civic, or the Caravan to the Odyssey and then come back and say that.
FThorn 8:16PM (11/22/2007)
How do you know his Dodge Caravan is much larger? One of mine has 2.4 Liters.
Ducky 9:39PM (11/22/2007)
Your Civic most likely has a 1.8 or lower engine. If you had an Si it would have been a 2.0 litre engine at best.
psarhjinian 8:32PM (11/22/2007)
Considering his Civic could have anything from a 1.2L to a 2.0 (most Civics, until recently, on the road are 1.6L non-VTEC), and that only the very bottom of the barrel Caravans came with the 2.4L (most came with 3.0L+ sixes), I think it's very likely that the van in question has at least a half-litre and probably much more.
tekdemon 5:47AM (11/23/2007)
Weird that you chose the dodge caravan, since I still have nightmares about driving a Chrysler Town and Country and attempting to get it to accelerate from 50->60MPH...while it was pretty hilarious to watch the speedometer stay constant while the engine roared loudly, I would not say that it is a great example of a torquey engine. Maybe your girlfriend's Caravan has an upgraded engine or something, but the base 3.3L V6 is a joke considering how friggin' heavy those minivans are.
psarhjinian 5:43PM (11/22/2007)
The screamer engines have a relative lack of torque at low RPM compared to engines of the same horsepower, but they're not really any more torque-deficient than any other manufacturers naturally-aspirated mill of the same displacement. You could call, say, the Fit's 1.5L torqueless, but that's because it's a 1.5L, not because it's a Honda engine. The Yaris, Accent/Rio and Aveo have about the same torque curves, and the Honda and Toyota at least get reasonable mileage.
This was an easy cheap shot to take at Honda back when they still made the non-VTEC 1.6 and competitors (Nissan, GM) were making engines with more than a half-litre of additional displacement. The 1.8 and 2.4 are more than competitive in terms of torque.
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EMC 5:44PM (11/22/2007)
actually, most honda engines have very good torque when compared to other engines of the same capacity.
Thats the whole idea of vtech/vvt/etc... better torque *and* better power.
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Ducky 8:07PM (11/22/2007)
The newer I4 Hondas produce about 80% of their peak torque at only 2000-2500 RPM, while producing some of the best fuel economy and low emissions numbers out of all production engines. If there is any reason why these vehicles seem to "lack torque", it's because of their small displacement.
Tricky dicky 5:46PM (11/22/2007)
Nothing world class about it. Especially given the vehicle which sports that engine runs like it is tied to a tree.
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Cameron 8:03PM (11/22/2007)
The RDX will outrun and outhandle a WRX.
FThorn 8:17PM (11/22/2007)
I've driven both. I'd like to see the RDX outrun/handle the WRX.
LOLWUT 8:49PM (11/22/2007)
Cameron, Stop doing PCP, it's really getting to you, the RDX is so slow, and will never out do one on the track, manly because of that 5 Speed, and the turbo lag, not to mention the fact that it's in desperate need of a diet, and some lipo.