Abstract

In this paper, research on hydrogen internal combustion engines is discussed. The objective of
this project is to provide a high efficiency means of renewable hydrogen-based fuel utilization.
The development of a high efficiency, low emissions electrical generator will lead to establishing
a path for renewable hydrogen-based fuel utilization. A full-scale prototype will be produced in
collaboration with industrial partners.

The electrical generator is based on developed internal combustion reciprocating engine
technology. It is able to operate on many hydrogen-containing fuels (e.g. H
2
, CH
4
O, NH
3
,
Biogas, etc.). The efficiency and emissions are comparable to fuel cells (50% fuel to electricity,
~ 0 NO
x
). This electrical generator is applicable to both stationary power and hybrid vehicles. It
also allows specific markets to utilize hydrogen economically and painlessly.

Introduction

Two motivators for the use of hydrogen as an energy carrier today are: 1) to provide a transition
strategy from hydrocarbon fuels to a carbonless society and 2) to enable renewable energy
sources. The first motivation requires a little discussion while the second one is self-evident.

The most common and cost effective way to produce hydrogen today is the reformation of
hydrocarbon fuels, specifically natural gas. Robert Williams discusses the cost and viability of
natural gas reformation with CO
2
sequestration as a cost-effective way to reduce our annual
CO
2
emission levels. He argues that if a hydrogen economy were in place then the additional
cost of natural gas reformation and subsequent CO
2
sequestration would be minimal (Williams
1996). Decarbonization of fossil fuels with subsequent CO
2
sequestration to reduce or eliminate
atmospheric emissions provides a transition strategy to a renewable, sustainable, carbonless
society. However, this requires hydrogen as an energy carrier.
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Proceedings of the 2002 U.S. DOE Hydrogen Program Review
NREL/CP-610-32405
Background

Electrical generators capable of high conversion efficiencies and extremely low exhaust
emissions will no doubt power advanced hybrid vehicles and stationary power systems. Fuel
cells are generally considered to be ideal devices for these applications where hydrogen or
methane are used as fuel. However, the extensive development of the IC engine, and the
existence of repair and maintenance industries associated with piston engines provide strong
incentives to remain with this technology until fuel cells are proven reliable and cost competitive.
In addition, while the fuel cell enjoys high public relations appeal, it seems possible that it may
not offer significant efficiency advantages relative to an optimized combustion system. In light
of these factors, the capabilities of internal combustion engines have been reviewed.

In regards to thermodynamic efficiency, the Otto cycle theoretically represents the best option
for an IC engine cycle. This is due to the fact that the fuel energy is converted to heat at
constant volume when the working fluid is at maximum compression. This combustion condition
leads to the highest possible peak temperatures, and thus the highest possible thermal
efficiencies.

Edson (1964) analytically investigated the efficiency potential of the ideal Otto cycle using
compression ratios (CR) up to 300:1, where the effects of chemical dissociation, working fluid
thermodynamic properties, and chemical species concentration were included. He found that
even as the compression ratio is increased to 300:1, the thermal efficiency still increases for all
of the fuels investigated. At this extreme operating condition for instance, the cycle efficiency
for isooctane fuel at stoichiometric ratio is over 80%.

Indeed it appears that no fundamental limit exists to achieving high efficiency from an IC engine
cycle. However, many engineering challenges are involved in approaching ideal Otto cycle
performance in real systems, especially where high compression ratios are utilized.

Caris and Nelson (1959) investigated the use of high compression ratios for improving the
thermal efficiency of a production V8 spark ignition engine. They found that operation at
compression ratios above about 17:1 did not continue to improve the thermal efficiency in their
configuration. They concluded that this was due to the problem of non-constant volume
combustion, as time is required to propagate the spark-ignited flame.

In addition to the problem of burn duration, other barriers exist. These include the transfer of
heat energy from the combustion gases to the cylinder walls, as well as the operating difficulties
associated with increased pressure levels for engines configured to compression ratios above
25:1 (Overington and Thring 1981, Muranaka and Ishida 1987). Still, finite burn duration
remains the fundamental challenge to using high compression ratios.

The goal of emissions compliance further restricts the design possibilities for an optimized IC
engine. For example, in order to eliminate the production of nitrogen oxides (NO
x
), the fuel/air
mixture must be homogeneous and very lean at the time of combustion (Das 1990, Van
Blarigan 1995). (It is subsequently possible to use oxidation catalyst technologies to sufficiently
control other regulated emissions such as HC and CO.) Homogeneous operation precludes
diesel-type combustion, and spark-ignition operation on premixed charges tends to limit the
operating compression ratio due to uncontrolled autoignition, or knock. As well, very lean
fuel/air mixtures are difficult, or impossible to spark ignite.


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Proceedings of the 2002 U.S. DOE Hydrogen Program Review
NREL/CP-610-32405
On the other hand, lean charges have more favorable specific heat ratios relative to
stoichiometric mixtures, and this leads to improved cycle thermal efficiencies. Equivalence ratio
(
) is no longer required to be precisely controlled, as is required in conventional stoichiometric
operation when utilizing tree way catalysts. Equivalence ratio is defined here as the ratio of the
actual fuel/air ratio to the stoichiometric ratio.

Combustion Approach

Homogeneous charge compression ignition (HCCI) combustion could be used to solve the
problems of burn duration and allow ideal Otto cycle operation to be more closely approached.
In this combustion process a homogeneous charge of fuel and air is compression heated to the
point of autoignition. Numerous ignition points throughout the mixture can ensure very rapid
combustion (Onishi et al 1979). Very low equivalence ratios (
~ 0.3) can be used since no
flame propagation is required. Further, the useful compression ratio can be increased, as
higher temperatures are required to autoignite weak mixtures (Karim and Watson 1971).

HCCI operation is unconventional, but is not new. As early as 1957 Alperstein et al. (1958)
experimented with premixed charges of hexane and air, and n-heptane and air in a Diesel
engine. They found that under certain operating conditions their single cylinder engine would
run quite well in a premixed mode with no fuel injection whatsoever.

In general, HCCI combustion has been shown to be faster than spark ignition or compression
ignition combustion. And much leaner operation is possible than in SI engines, while lower NO
x

emissions result.

Most of the HCCI studies to date however, have concentrated on achieving smooth releases of
energy under conventional compression condition (CR ~ 9:1). Crankshaft driven pistons have
been utilized in all of these previous investigations. Because of these operating parameters,
successful HCCI operation has required extensive EGR and/or intake air preheating.
Conventional pressure profiles have resulted (Thring 1989, Najt and Foster 1983).***

In order to maximize the efficiency potential of HCCI operation much higher compression ratios
must be used, and a very rapid combustion event must be achieved. Recent work with higher
compression ratios (~21:1) has demonstrated the high efficiency potential of the HCCI process
(Christensen et al 1998, Christensen et al 1997).

In Figure 1, the amount of work attained from a modern 4-stroke heavy duty diesel engine is
shown at CR=16.25:1. The results indicate that under ideal Otto cycle conditions (constant
volume combustion), 56% more work is still available. This extreme case of non-ideal Otto
cycle behavior serves to emphasize how much can be gained by approaching constant volume
combustion.

Engineering Configuration

The free piston linear alternator illustrated in Figure 2 has been designed in hopes of
approaching ideal Otto cycle performance through HCCI operation. In this configuration, high
compression ratios can be used and rapid combustion can be achieved.
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Proceedings of the 2002 U.S. DOE Hydrogen Program Review
NREL/CP-610-32405
10
5
10
6
10
7
10
8
0.00 01
0.0 01
10
5
10
6
10
7
10
8
P
r
e
s
s
u
re
(P
a
)
Volume (m eters
3
)
Cons tant Volum e Com bustion
D iesel E ngine
100 %
64 %
56 % More W o rk In Constant
Volume C ombustion C ycle

Figure 1 Modern 4-Stroke Heavy Duty Diesel Engine





Intake
Exhaust
Alternator Windings
Magnets
Piston
Cooling Fluid
Turbocharger
Exhaust Valves
Delivery Tank
Intake Runners
Figure 2 Free Piston Linear Alternator

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Proceedings of the 2002 U.S. D