Mercedes Benz E-Class 800
The author's hypothetical Mercedes Benz E-Class 800H could be driven at over 100 mph on the autobahn and deliver 60 mpg fuel-efficiency at normal highway speeds, while offering 30 miles of engine-off range in the city plus impressive acceleration, using an engine DaimlerChrysler already have in mass production.

So You Want a Diesel-Hybrid, Do You?

Why you will want a gasoline-powered 'strong' hybrid, and why diesel-hybrids won’t make it in North America.

By Chris Ellis

Editor's note: The author is the CEO of The PowerBeam Company, based in the UK. His company has no plan -- currently -- to produce a PowerBeam-based E-Class demonstrator (they are concentrating on a Jaguar XJ-based machine), but they are prepared to give DaimlerChrysler every reasonable assistance when they realize they’d like to produce one themselves.

Diesel-electric hybrid cars make no real sense in North America, and not much elsewhere. Period. That's a pretty strong statement, but suspend disbelief, read the following and then make up your own mind.

Below is a table comparing most of the EU4 emissions-compliant models from the full Mercedes E-Class sedan range available in the UK and Europe. All are in 'Elegance' trim (except the E 55), and all are fitted with automatic transmissions. The information is from www.mercedes.co.uk

E 200 K £28,660163 bhp 141 mph 43.5 mpg 214 g/km 1570 kg 9.9 secs
E 220 CDI£29,895150 bhp130 mph 39.8 mpg188 g/km1610 kg10.1 secs
E 320 CDI£35,505204 bhp151 mph44.8 mpg194 g/km1735 kg7.7 secs
E 240 V-6£31,950 177 bhp145 mph25.5 mpg250 g/km 1570 kg10.1 secs
E 350 V-6£35,770272 bhp155 mph29.1 mpg231 g/km1680 kg 6.9 secs
E 500 V-8£44,150306 bhp155 mph24.4 mpg258 g/km1725 kg 5.9 secs
E 55 AMG£63,450476 bhp155 mph21.9 mpg 310 g/km1635 kg 4.7 secs

The reason for using the UK range as an illustration is that it offers more E-Class variants than are available in North America. The prices are retail, including sales taxes, etc. The g/km column shows the CO2 output in grams per kilometer. The kg column is the weight with fuel, a driver and a small amount of baggage. The last column shows the time to 62.5 mph (100 kph) from rest. By choosing a range of vehicles that share the same body shape and trim level and therefore have near-identical aerodynamic characteristics and similar weights, it's possible to separate out the effect of engine size from the other key variables that determine fuel consumption. Large differences are revealed, which provide strong evidence to support the case for Strong Hybrids, characterized by radically reduced engine size and powerful energy storage systems.

The E 200 Kompressor version of the E-Class, unavailable in North America, is powered by a forced induction gasoline 4-cylinder engine of only 1.8 liters capacity, yet it still offers a top speed of over 140 mph, fast enough for most normal and abnormal purposes. As the UK has a punitive CO2-related tax on company cars there is a market for an E-Class with an even lower CO2 figure, which is met by the turbocharged 2.2 liter 4-cylinder diesel engine in the E 220 CDI. Much more appealing in terms of performance and quality of delivery is the 6-cylinder diesel engine in the E320 CDI, which has recently been made available in the US. In Europe there is also the gasoline E 240 V-6, which accelerates more slowly than the $5,000 cheaper E 200 K, produces more CO2 and uses more fuel. It's essentially for those who value the smoothness and quietness of a gasoline six but not the acceleration of the E 350 and apparently don't care much about Global Warming. The 350, 500 and E 55 have all been available in the US for some time.

It is amusing to note that, in the land of the unrestricted autobahn, there is an additional model at the bottom of the range, the 125 mph 200 CDI, which is over $3,000 cheaper than the 220 CDI. This, of course, is much too slow and cheap to be sold in the UK or the US. However it is likely to be replaced in its principal role of 'übertaxi' by the new 200 NGT, essentially a natural gas version of the 200 K producing less than 180 g/km of CO2 and costing much less to refuel.

The mpg figures shown are for the extra-urban element of the official New European Drive Cycle, and are measured in Imperial gallons, not US gallons. However, for the purposes of this discussion it is the relative mpg figures that are important, rather than the absolute values. Ideally, the fuel consumption figure of most interest in this exercise would be for a steady 70 or 80 mph on the level with no wind, but the nearest available and reliable numbers are those shown. As the NEDC extra-urban numbers are likely to show less variation than the figures for a 'steady 70' they are probably conservative for our purposes.

From the perspective of developing a Strong Hybrid version of the E-Class, the urban and combined mpg figures of the existing vehicles are irrelevant as a powerful kinetic energy store will produce good mpg figures in the city, almost irrespective of engine size. But, as many Prius and Insight owners have confirmed, fuel consumption at cruising speed on the freeway has rather more to do with aerodynamics and engine size/efficiency than the efficiency of the hybrid system. However, the right hybrid system can allow a level of engine downsizing that will both satisfy the market's lust for acceleration and make a contribution towards the strategic goal of 'oil independence'. This paper will now demonstrate how this might be achieved using an engine which already exists in DaimlerChrysler's product line, leave alone what may already be in development.

An initial comparison of the 200 K and the 220 CDI might make one wonder why anyone would buy the diesel CDI. We are being asked to pay $2,000 extra for less acceleration, more noise and apparently an almost 5 mpg worse fuel consumption figure, leave alone a lower top speed. The only gain seems to be a small drop in CO2 output. So why does the CDI sell in considerable numbers? Essentially it's because of the superior efficiency of the diesel engine in city traffic. The NEDC urban figure for the CDI is 29.4 mpg versus the 21.7 mpg of the 200 K, and in actual use the difference is likely to be even greater. However, in a full or strong hybrid the diesel engine's running-cost advantage in the city is almost completely eroded, particularly if the hybrid is a plug-in.

It is worth noting that in upgrading the CDI engine from the EU3 to the EU4 emissions standard the extra-urban fuel figure fell from 44.8 mpg to the 39.8 mpg shown in the table. This is one reason why experts such as Porsche and Ricardo are taking the position that further development of gasoline engines will close down the 'efficiency gap', and that the next generation of highly-efficient gasoline engines will remain less expensive to build than the equivalent diesels. For more detail, see the article from Ricardo's Quarterly Review. Some European manufacturers continue to claim that small diesel cars will be more economical to run than small gasoline hybrids. But what happens if we combine some of the elements of LBDI (we don't need them all) with start/stop, etc? Better economy than diesels at ALL speeds, at similar cost. Then factor in the cost, tax and emission advantages of natural gas over diesel or gasoline.

Once the French government wakes up to the fact that encouraging diesel rather than gasoline plug-in hybrids will increase the cost of French oil imports and French cars, and that the lower tax on diesel is losing the government substantial revenue, then perhaps it will comply with the European Union's collective decision to harmonize gasoline and diesel taxes. This will then remove the last, anachronistic, incentive to develop and promote diesel cars. With low-cost, emission-free electricity available everywhere in France overnight, the French government is curiously slow to exploit the major economic and emission reduction advantages the French people have already paid for and installed. Could it be tax related?

If diesel engines were as wonderful as their proponents claim, why have there been no successful military or commercial diesel aircraft, despite the many efforts since the 1920's? Surely fuel consumption is even more important in aircraft than in automobiles? So why (almost) no diesel aircraft? The (over) simple answer is that diesel engines only shows real advantages in light vehicles at low loads, which are almost totally absent in aircraft and hybrids. If Toyota is right, and they probably are, that almost all road vehicles (FCVs included) will eventually have a temporary energy store to save the energy otherwise wasted on braking and support downsizing of the engine or fuel cell and engine-off running, then diesel hybrids will become historical curiosities.

After that polemic, let's return to the objective, specifying our ideal hybrid E-Class. The obvious power unit is the 163 bhp 1.8 liter 4-cylinder gasoline engine, which meets all the normal hill climbing and towing requirements and can deliver a top speed of over 140 mph in the E-Class body. Now let's say we would like to match the acceleration of the E 500 up to 62.5 mph, which is achieved in only 5.9 seconds, four seconds faster than the 200 K. The E 500 needs 306 bhp to achieve this and overcome its 155 kg weight penalty, most of which we can claim as our 'hybridization weight budget', even allowing for features which are standard on the E 500 but are extra cost (and weight) options on the 200 K.

The kinetic energy inherent in a 1725 kg E-Class travelling at 62.5 mph is 185 Wh. This is then the most that could possibly be recovered by a 100% efficient regenerative braking system. Of course, the Second Law of Thermodynamics dictates that we will never get 100%, but let's assume that we are able to return at least 60% to the driving wheels on subsequent acceleration. This would mean that the engine would only need to contribute 40% of the energy required from the conventional engine, at about 40% of the power. The kinetic energy store must provide 60% of the power as well as energy under these minimal conditions. However, to allow the vehicle to push swiftly up to European speed limits and also keep the average sane German happy, we might want to set an objective of reaching 100 mph as fast as the E 500. This won't require more power but it will require more energy capacity. The inherent kinetic energy in 'our' E-Class at 100 mph is almost 500 Wh, so we might take that as the available energy capacity of our ideal device, with a power target of at least 200 kW to ensure 'our' hybrid can embarrass an E 55.

The additional energy from the engine will serve to overcome all the losses in the system, particularly the aerodynamic losses that build up rapidly beyond 60 mph. Of course, in the real world it is a very fortunate driver who has the opportunity to go from rest to 100 mph. However, our German friends will be very interested in the 70 mph to 120 mph time, which will beat almost anything because the kinetic energy system naturally acts as a constant torque, rising power device, ALL the way up, unlike a typical electric motor. This will deliver seriously quick acceleration on the autobahn, when the opportunity presents itself. The rest of the time, impressively frugal fuel consumption. Cake, anyone?

The vehicle itself should be based on the E 500, without the V-8 and its expensive transmission but with all the standard toys and with a similar sticker price. In the UK, the retail price gap between the 200 K and the 500 is roughly £15,500, of which we'll assume £3,500 is down to 'standard extras' on the 500. In dollar terms, this leaves around $20,000 associated solely with the larger engine, transmission and brakes. Our '800 H' doesn't need the larger brakes because it has the same top speed as the 200 K and regenerative braking should more than cope with the extra weight. Of course, we must assume that DaimlerChrysler makes a better margin on a 500 than on a 200 K, and we know that far too much of the $20,000 gap is down to UK taxation policy. Let's be ultra-conservative and claim there is a $7,000 cost budget available for the hybrid stuff, with the 200 K engine and it's associated automatic transmission already accounted for. $7,000 buys a lot of hybridisation. It should also pay for a 30+ mile plug-in battery in addition to the kinetic energy store, which will make the vehicle effectively a ZEV in the city, and no one will hear that your engine has only four cylinders. If the badge says 'E 800 Hybrid' that will be enough to impress most neighbors!

As a cross check, the current price gap between a US spec E 350 and an E 500 is over $9,000, and you will need to come up with a further $23,000 to enjoy the faster acceleration of an E 55 AMG. Check out www.mbusa.com

Now, imagine you've just been appointed to a seat on the board, and it comes with a company car, provided the sticker price doesn't exceed $90,000. You visit your local Mercedes dealer and are confronted with a conventional S 500 and a hybrid S 800 H which is quicker to 100 mph, gets 60 mpg driven gently on the freeway, and has 30 miles of engine-off range in the city. The only apparent shortcoming of the hybrid is that it can't cruise continuously at more than 130 mph. As if. The sticker price on the S 500 is $85,000. The sticker price on the S 800 H, complete with the optional extra plug-in battery pack, is $88,000. Which one are you going to pick? In the UK, making the wrong choice will cost you several thousand dollars in extra 'company car' tax every year. If you buy it yourself, remember who's paying for the fuel. Of course, if most of your driving is out of town, pick the natural gas version of the S 800 H, which will cost even less to run on the autobahn or freeway, and re-fuel it in your own garage. Which car will have the higher residual value?

This paper doesn't attempt to explain how 'strong hybridization' will be achieved; that's for another day, in another paper. What it should have done is demonstrate the potential. As a final example, consider what might result if the 'full hybrid' DC/GM system already shown in an S-Class concept vehicle is put into an E-Class and is compared with 'our' E-Class hybrid. The DC production version might use the 3.5 liter V-6 of the E 350 as shown in the concept, which gets only 29.1 mpg on the highway in the standard E 350 versus the 43.5 mpg of the standard gasoline 200 K. Both these numbers will rise in the hybrids, but a considerable gap will remain. In the city, the plug-in 'Strong Hybrid' will consume no gasoline most of the time, and obviously less than the DC version on those rare occasions when it needs to fire up.

Imagine now you are a world-famous Hollywood star and you want to trade in your Prius for something just as economical but with more space and better performance. Money is irrelevant but image is everything. Are you going to pick a Lexus 400h SUV that can only manage 50(?) mpg at a steady 70 and takes 7.3 seconds to reach 60 or a Jaguar XJH that can do it in less than five seconds, yet goes 70 miles per US gallon at a constant 75?

In summary, mild hybridization is just a first step and a PR expedient, in all but the smallest cars. Full hybrids are better but 'Strong Hybrids' are best, particularly with 30+ miles of plug-in capacity. In defense of Honda, the Insight is a baby 'Strong Hybrid' in need of a kinetic energy store and a back seat. With that, Honda could wipe the floor with the Prius, particularly in Europe.

What the US government and people need to understand is that oil independence is readily achievable, without the breakthroughs required to deliver the much vaunted 'hydrogen economy'. But does your government really want it?

As Mark Twain would have said - "the death of suburbia has been greatly exaggerated".

Times Article Viewed: 29088
Published: 10-Feb-2005


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